Problem of transistor thermal runaway in parallel connection
If it is acceptable in your application, you can add resistors in series with the transistors on the emitter pin.
The transistor that carries the higher current will see a Vbe decrease because of the additional current flowing in the emitter resistor, therefore the runaway effect will tend to balance out.
The drawback of this approach is that you have additional power dissipated in the emitter resistors, but with 500 mA for 3 seconds, as you mention in the post, I am guessing that is not a big problem.
I also see that you mention that you cannot find a suitable FET for this application, but perhaps you can find a bigger BJT rated for higher currents, that make you feel a bit safer. It might even be possible for you to switch to an N device and put it on the ground side of the boost converter, but that depends on a number of different things.
If the rationale here is that you have some 2N2907 in the drawer and you want to use them, then I suggest the emitter resistors.