MacBook Pro with two external monitors : one is flickering
This problem is not due to the computer not being able to serve the necessary amount of "fps". Instead this is a hardware problem.

VGA is an analog signal. If the signal is distorted or interfered with, it could problems such as flickering. The flickering stops when you disconnect the first monitor - which indicates that this monitor (and cable) is the source of interference.

I would suggest replacing the VGA cable with one with a ferrite bead instead. Another option is to replace the VGA adapter with a DVI adapter and connect to the monitor using a DVI cable.

If possible you could also try to the cables seperate from each other physically (i.e. get distance between them) - but it is difficult as they need to plug in to the computer very near each other.