From nvidia's FAQ: Can I mix and match graphics cards with different sizes of memory?
No. For example, an XXXGT 512MB cannot be paired with a XXXGT 1GB in an SLI configuration.
So no, the 2g one will not SLI with the 1g one.
It's not recommended by Nvidia because there are a few cards where that rule applies but many people have SLi'd cards with different amounts of RAM with the only ill effect being that the system only sees both cards as having the lower amount.