There's no need in osx.
Assuming both devices are compatible with aggregate device, you can use that.
It's a virtual interface that contains two or more hardware interfaces.
Go to audio/midi setup in utilities, create an aggregate device and select your two interfaces in the aggregates list.
Ensure that they're both set to the same specs too.
Now your recording software should see 'aggregate device' as if it's just one big piece of hardware, meaning you have access to every input and output.
Numbering can be a bit tricky; It may take some trial and error to figure out the order.