Yea, I'd like to see Pacino do both characters as well. He'd be great I'm sure.
However, I can't see the "characters" of MC and TM connecting at all. Two totally different kinds of criminals. MC had more "class" (if it's even possible for criminals to have class), than Tony Montana. Now who would be the one to "whack" the other first? That might be interesting.
It's my guess that if this scenario were believeable, MC would have the edge simply because he was patient, didn't talk much and knew how to "keep his enemies close."
On the other hand, Tony Montana could, on a whim, start blasting when least expected and take everyone by surprise.

My money would still be with MC tho.
Don't know if this is were you wanted to go with this. Oh, and for the record, I didn't think MC's dialogue in the script you posted sounded like MC, although TM's sounded like SF dialog.
I guess what I'm saying is, although these characters would never get along, let alone be friends, it's an interesting concept.
TIS