As Emerald says, any given instance of Windows will only use ONE internet connection exclusively, ignoring all others.
However, there is a twist here. Windows 7 Ultimate supports XP Mode. I haven't used XP Mode personally, but if it's like most VMs (virtual machines), you can use one internet connection on the host OS (Win7), and pass the other internet connection to the guest OS (XP).
IOW, if one instance of Windows isn’t enough, just create another! No, it's not absolutely ideal, maybe even a bit clumsy, but it should work. I've done it myself w/ Oracle VirtualBox (which has the ability to seamlessly integrate the guest OS into the host OS, very cool).