?

convert the second adat on the desk to analog (adda8000)
and use the 8 analog i/o from the 003

What mixer is it? If it's something like a Yamaha 01V96, you could mix pairs of channels to each of the 8 auxes which you then output via ADAT to the 003.

Thanks for these ideas, both of those should work. the mixer is a Korg 168RC. I'm wanting to configure this to record 16 simultaneous tracks going through the mixer and then to the 003. Plan B is to go into the 8 inputs on the back of the 003 and 8 via adat from the mixer.