The short answer is yes.
Here is the description to the Backstab skill from Rikimaru
If Riki attacks from behind, bonus damage is applied based on his current agility.
That said, all attacks from behind will do extra damage that can be calculated (ignoring armor resistance) by adding the ammount added to the base damage from your character.
Here is some information about how Critical Strikes work:
Critical damage is physical damage and reduced by various types of armor. The red number shown by critical strike is displayed before reductions (this is why illusions don't show very low red numbers). Critical strike does not work against buildings and is subject to pseudo-random distribution. Because critical strike multiplies the attack damage of a unit, it is a scaling ability (unlike the damage of Monkey King Bar's mini-bash ability).
Backstab and critical strike can proc in the same attack. Period.
What's not clear here is when will the critical strike bonus be calculated.
The backstab skill adds bonus damage to your atacks based on your agility, and critical strikes have a chance of x to multiply by y your atack damage.
Lets say critical strike damage is the last parameter to be calculated. That would lead us to something like this ( I picked the values )
Attack Base : 50 , Backstab Bonus : 40 , Critical Strike Multipler : 2
Total Damage = (50 + 40) * 2 = 180
On the other hand, if Backstab bonus is added after the Critical Strike sums up, we would end up having :
Attack Base : 50 , Backstab Bonus : 40 , Critical Strike Multipler : 2
Total Damage = (50*2) + 40 = 140
TL:DR
Whether the Critical Strike is calculated after or before Backstab sums up, is not clear, but they CAN proc in the same attack.
Outdated answer
This is explained nicely on the Gold page of Dota 2 Wiki:
Gold can be split into two categories:
- Reliable gold – Any bounty you get from hero kills, Roshan, and global gold from towers is added to your reliable gold pool.
- Unreliable gold – Everything else (creep kills, neutrals, etc).
The difference between the two (except from how each is earned) is how
each one is spent:
- Dying can only take away gold from your unreliable gold pool and not from your reliable gold.
- Buying items uses up your unreliable gold first before using your reliable gold.
- Buyback uses reliable gold first.
In the end you still have a fixed amount of total gold (you just add
the two together).
The purpose of having these two categories is to encourage ganking and
tower kills that accumulate the reliable gold pool, which is gold that
cannot be lost from dying.
I tried to "research" this further by killing different things in a practice game.
You get reliable gold for killing an enemy hero. For killing Roshan everyone on your team gets 200 reliable gold. If an enemy tower is destroyed, everyone on your team gets reliable gold (and you get additional unreliable gold if you got the last hit on it).
Killing creeps or the courier gives you only unreliable gold.
At the start of the game you get unreliable gold, and then 1 unreliable gold every 0.6 seconds.
Abilities that give gold (thanks to Samthere for mentioning them):
Best Answer
This is because both are unique attack modifiers so they won't stack.
You are basically giving up their main purpose (either the armor reduction or life steal depending on which item is older).
You can still get the benefit of the passive and activation ability but those are not nearly as cost effective.