Question

two inverters connected in series act as a bistable multivibrator . how????

Answer 1

Bistable multivibrator means it has tow states which are stable.you need to trigger externally to change them.your ckt works like a bistable multivibrator because its input states are changing from 0 to 1 2 times because of these inverters which is what bistable exactly do.
Diff. between monostable and bistable is that in bistable you need to trigger for each stage to change and in monostable you need to trigger once for 2 states.