What does your dad use his computer mostly for? i will sign it as long as your Gaming needs outweigh his needs for the bigger monitor :p
Yea 12" is absoloutly crap for gaming though, my 19" Widescreen was only £145..
But if i was your dad, i would say, hell no, go buy you own, because going from a 20" monitor to a dual 12" would be hell, its like 1/10 your going to get that monitor, highly unlikely.