Matrix

This nice bit of software can combine multiple audio interfaces into a virtual audio interface (or multiple virtual interfaces if you whish) on windows.

Now i can finally feed Nuendo from all my interfaces at once. You can even route signals between different classes of drivers, so e.g. i can combine two ASIO interfaces and a gaming-headset with its non-ASIO driver.

I use it exclusively for routing, but every connection also has a volume setting, so you could use this for some static mixing.

The preset system was quite confusing to me, so if you want to use that search for the explanation in vb-audios forum.