Twin/Tandem's arent available, but double pole breakers should be.

Okay, but one more thing...
If Siemens breakers fit, why would they be dangerous? At worse, they might
arc if they do not fit perfectly, but an arcing breaker in a box shouldn't
start a house fire. And if there is such a problem, wouldn't it be pretty
noticable?
Please explain.
I have a Sylvania box with a bunch of non-Sylvania breakers, and if there is
a problem, I would like to understand it.