I don't believe you can.
My first audio interface, which I still have, but no longer use, is the earlier version - the UA-25, which is almost identical and I'm reasonably sure you have to pick one or the other, but not both.
I think once that digital button is pressed, that's all you get.
The most you can record is one stereo pair input [L & R], or 2 single line inputs, [L & R], and I'm pretty sure that the digital in is considered a stereo pair, therefore counting as your 2 inputs right then & there, with no way to split them up.