Will I be able to watch 2 VBoxes simultaneously? Will it affect the picture quality?

There is no problem with watching two TV's with two VBoxes simultaneously. However, please ensure  that you are subscribed to a fast enough internet speed in your household to power good picture quality. Each TV channel would consume 4 Mbps, therefore two simultaneous channels will consume 8 Mbps. You will still need to have at least 1 Mbps in reserve.