GAMING PC GRAPHICS CARD

Hi. I'm aware I may seem unprofessional from the title, but I assure you I'm anything but. I just have a simple question (well not simple to me) about graphics cards. I currently have an NVidia GeForce GT 640 (or 650 not sure).

I was thinking about purchasing an EVGA GeForce GTX 750 Ti.

Would it be possible to tell a game "I want you to use video memory from this card" on whichever I chose? For example. If I wanted Minecraft to run off my older one, and Watch_Dogs for the new one, would it work?

The main reason I'm asking is because I really don't want to sell my current card or have it go to waste

No it wouldn't you can't have two cards that are different only two that are alike sorry just get the 750ti.