On Death
You talk a fair amount about character creation being important to you, you create detailed back stories and you take time to shape each character. It seems natural to me, then, that you should also try to make sure that character deaths are important, impactful, and meaningful.
This might require talking to GMs, and adjusting the Social Contract at your tables, but it's a small change and I suspect most tables would be receptive to it. It also might require that you shift you perception a little bit, but again I don't think its a reasonable small shift to make.
Death should be avoided, but sacrifices are respected
What I mean by this is that whenever a character takes a dive, it should be a moment that is remembered and respected. In the scenario against the guy who always seems to crit, view him not as lucky, but rather as incredibly mighty. This foe has managed to cut to prices characters who have slain trolls, defeated liches, or saved villages from rampaging orcs. When he was finally defeated, at great cost, the weary remains of the party dragged themselves back to town where perhaps a shrine was raised to honor the loss.
This kind of respect and memory can be augmented by a gm giving dying characters a little extra liberty in their deaths. Once the table knows that a character is dying. I like to let the character act out a slightly elongated fight scene that ends in their demise. This might mean that a barbarian takes a sword through the chest, but pulls it out and throws a punch (mechanically no effect) that sends the baddy reeling before he collapses to his knees.
Another way, that doesn't require a change by anyone else at the table, is to view death as not an ending, but a new adventure. The loss of a character gives you a chance to view the rest of the campaign through a new lens, and to fill in gaps that may have only become evident once you're some ways into a campaign. In my personal view, I find that making new characters can be really enjoyable, especially in regards to being able to mix up a party dynamic that has gotten a little too comfortable.
On Failure
Something that may help in regards to non-death failures would be to consider that weaknesses are quite possibly the most interesting part of any character. It gets boring to watch a character always succeed at things. We even have a name for such a character: the Mary Sue. How, then, do we get used to the fact that our characters aren't always capable of everything they want to do? A tactic I like to use is to take gambles where it's more-likely-than-not that I'll fail. This serves two purposes: it gives my characters a depth that failure can cultivate, and it helps me get used to the idea of my characters failing. That way, when I engage in an action that I actually expect to succeed at, and I fail, I can adopt the mentality I've become accustomed to before, and continue on without bad feelings.
Some systems actually encourage this kind of risk taking, and might be worth looking into. One I personally have experience in is Anima: Beyond Fantasy, which rewards you with XP at the end of sessions where you succeeded on skill checks you were likely to fail (therefore encouraging you to take risks to earn that bonus XP).
This situation may not be saveable, but here's an option
Sometimes, personality conflicts can't be solved. This may or may not be such a case and I commend you for your efforts on trying to act as the healer.
Don't try to solo this one. Recruit allies: the other three players
Given what you have already tried, and that you are losing fun due to this constant friction, it is reasonable that you call on the other three players (not characters) in your group to step up and contribute to solving the problem. All six of you are in this together.
Two players with diverging expectations and a personality conflict
Between you describing a little bit of My Guy Syndrome for Colin, and a little bit of "I am so special" for Sean, and your comment observations that the other three players are alternately amused and concerned ... there may be more parties to this friction than just Sean and Colin.
There are, in total, 5 players in the group. Aside from Sig and
Colt, the party also consists of an illusionist wizard, a college of
lore bard (who makes a very effective healer), and a pact of the
blade warlock. Everyone else gets along and has been watching these
events with a mix of amusement and concern.
If I'm going to lose one player, it will definitely be Sean. I would
like to limit answers to anything that would allow me to retain
both. Sean is already teetering on the edge of leaving the group, so
it would be easy enough to just let him go.
Ask the other three players to help you rather than just act as a Greek chorus for the drama being played out before them. In discussing solutions to this problem with these three players in an "away from the table" session:
You need to be clear that you want them to help Sean integrate better with the group, since losing Sean is the problem you are trying to solve. (i.e. Your aim is to keep Sean in the group.)
You need to be clear that you have exhausted your people handling skills, and that you need their help in reaching Sean from a position of a peer, since your role as DM brings with it some possible "anti-authority" issues.
You need to be clear that DM's are allowed to have fun to, and that this group dysfunction is harming your fun. If they find playing at your table to be fun, you are asking them to "help me help you."
You need to be clear that they need to be honest with you: if they are happy to let Sean go, then they need to be clear about it, and be clear about it to Sean. If their amusement that you noted is generally at Sean's expense, this whole set up can regress toward a form of clique-based bullying. I am pretty sure that you don't want that to happen.
If they are interested in keeping the whole party together, then as players appeal to them to comment, rather than to just watch, when the see tension building up and to act to divert or preclude a player-on-player confrontation. Don't just sit there and take enjoyment in the other two getting into it again. The point is to defuse a budding player-on-player event.
Bottom Line
There are more peoples' fun at stake than just one: it's the fun of all six that is at stake. Appeal to them as stakeholders in the fun for a contribution to the effort, or, if point 4's answer is "we can play this without Sean, he's a pain" then that leads to a serial encounter. If Sean is that incompatible with this group, the group as a whole needs to be honest with him. Dumping this all in your lap is hardly decent, insofar as group dynamics is concerned.
Best Answer
I've never had the same character as another player but I have had characters fill the same niche. The best advice I can give in that case is to play off of that niche. When it's just one character who has a certain skill set, it's really easy for anything that uses that skill set to automatically be your job with no further discussion. But if you have someone to collaborate with you can actually start roleplaying out that niche and sucking the rest of the party into it.
In a GURPS game a few years back another player and I were woodsmen. We had different abilities in combat, but both stocked up on movement and survival skills. The end result was that we were really invested in movement skills and made the party use them. We were also highly competitive about it. Every time we were confronted with a a cliff face we'd have a climb race.
Basically by playing off each other we were able to make more of the game about our niche. It was okay to share the spotlight since together we had more command over that spotlight.