Hi! It is very possible to use multiple monitors with SA-MP. Does GTA freeze in the not responding and then crashing sense or do you mean it just not moving while you're working on something on the other monitor? If the first it is a common issue with GTA. Some things that might fix it:
Running it in a compatibility mode. Like winxp, win7, etc. (Right click on gta_sa.exe, properties, compatibility)
You could also try to run it as an administrator.

If these don't work try looking online for more solutions.