There is a way to make an aggregate device in Windows, but it relies on using non-ASIO drivers so your latency will be intolerably high. And then there's clock sync, which the UR44 doesn't have a way of controlling. Honestly, I'd sell both UR44s and buy a single UR824 to meet your needs.
I guess you could use one UR44 as a primary interface, then turn on direct monitor on the other and use that to pass through up to 2 channels from the 2nd UR44 to a couple of line inputs on the primary UR44. But that only adds up to 6 channels and not 8. (Actually I don't see a way to do that either...there's no stereo monitor option, and no way to pan the 2 inputs apart, so both inputs would end up passing through both outputs). I'd really just scrap the UR44 and go with something that natively supports the number of channels that you want to record simultaneously.