I suspect that part of the problem is that the two base-emitter junctions have slightly different Vbe values.
This matters because the Vbe voltage is two things. It is the threshold at which the transistor starts to conduct, but it's also the level where the junction acts as a zener diode.
So, the transistor with the slightly lower Vbe is clamping the voltage at a level to low for the second transistor to turn on.
As has been said, each transistor needs it's own resistor.