I'm not sure if this is the rule-of-thumb you are looking for, but you need to either degenerate with emitter resistors (simple) or control relative base currents (harder) so as to avoid having one transistor conduct most of the current due to its Vbe getting lower because it is hotter because it is conducting more current. (You want to get the gain of that partially thermal feedback loop enough below 1 that the parts share current about equally.)
The current hogging problem is almost nonexistent with MOSFETs because of their positive TC of channel R. For similar reasons, they are far less prone to second breakdown effects too. I would look into using them.