How far do the others get ? If you hit [ESC] as they boot you should
get a screen with options for older kernels and also recovery mode.
Try either an older kernel or recovery more then purge the NVidia
drivers using Synaptic Package Manager and then let the system get the
latest driver automatically or starting the Additional Hardware wizard
directly.

Not sure how it would work with two cards but if you have only one
monitor be wary about switching between them with the system live as
this may cause damage to one or more card - hot-plugging monitors
isn't recommended