2 modems in single PC?

I am using Windows XP, I just want to know if i will install 2 modems in my PC and connect both with the 2 tel. lines. And i have no any multilink ISP account.
If i connect one modem with one ISP and 2nd modem with another ISP,
Then What modem will be used for using internet.
Either both will work in NLB fashion or nothing will work.
And also tell me may it will provide me Backup connection. If one modem fails then 2nd is already working.
Pls help.