Is it possible to link/use two separate audio interfaces?

For example -- I have a ton of analog gear, and thus use two Echo AudioFire 12's linked up and clocked together to essentially become an "Echo AudioFire 24". The AudioFire's only have Analog Inputs and Outputs, and MIDI IN/OUT. There's no SPDIF or AES. I want to get the new RME UFX II (), which has 30 Input / 30 Output channels, but only 12 Analog IO -- which is not enough for all of my analog synthesizers to play simultaneously. What do I do? Is there a way I can utilize the 24 Analog IO from the AudioFire's, annnnd the RME UFX II AES digital inputs for my Digital gear in Reaper? Or do I need to use ADAT to get more analog channels? Ideally, I don't want my old interfaces going to waste though!

On a Mac you can create an aggregate device and combine inputs and outputs for any connected audio source or sink to create a virtual interface. I understand AISO allows the same thing but I don't know how that works.

Very interesting, never checked out ASIO Link Pro before, looks like that could come in handy as long as the latency isn't too bad. Looking at the application window seems it's only 25ms which isn't too bad, however. Haven't managed to get it to work on ASIO4ALL yet but should give it another shot.