Related to but different from this question, I have two three-way switches in my house, which are in multi-gang boxes at the top and bottom of the staircase and control lights at the top and bottom landings. I know for a fact, having fixed the "hot" side of the wiring to the three-way switches before, that the switches (along with others in both boxes) are fed by different circuits. However, all of the white neutrals in each box are twisted together into one bundle and wire-nutted; this predates any work I did on the circuit, but I don't know if this is the work of the original electrician or the "handyman" whose work I've been fixing since I moved in.
As these joined neutrals include the travelers of both three-way circuits, the neutrals from the nearest light to each box, and the neutral heading back to the panel from each box, these two circuits' neutrals have multiple junction points that tie the neutrals together.
Is this dangerous (from a fire hazard perspective)? My Google-fu brought up some similar questions, but the answers were all along the lines of answers to the related question in the link: "a shared neutral is bad". However, this isn't a "shared neutral" as in one white wire carrying the load from two circuits; this is two white wires carrying the load from two circuits, but the two white wires are both carrying the load from either circuit. So, to my way of thinking, there's plenty of copper to get the power back to the panel without overheating. And obviously there are no AFCI/GFCI breakers in the house (plenty of GFCI receptacles), because everything on both circuits works fine (they'd trip as soon as I turned a light on otherwise). The only concern AFAIK would be shock safety, as you'd basically have to make sure both of the breakers were off before working in either switch box (something I would do anyway; I wouldn't want ANYTHING hot in either box if I were messing with these switches, regardless of whether the neutrals are separate or not).
EDIT: picture == word*1000:
Best Answer
EDITED FOR CLARIFICATION:
If this is wired as you have drawn it, then it will not be a safety issue. The additional neutral will only share the current in both circuits. Because both circuits are fused for 15A, each neutral will only see a maximum of 15A. This is providing that both neutrals are solidly connected! If one were to become loose or disconnected then the other can potential see the full load, 15A*2=30A.
If anything else is fed from either of those breakers, it becomes a whole other issue! I recommend you wire it the right way. However to answer your question, I don't see it as a safety issue if that is the only circuit on those breakers.
With either breaker off, that circuit will be isolated from the energized circuit. The only common path between the two is the shared neutral. If the energized circuit were to draw the breaker maximum of 15A, the de-energized neutral potential would be at maximum only a few hundred millivolts, not posing a safety risk.
Also it is not against code to have more than one feed in a jbox or switch box.