Just saw this on the Bose forum.
Great Idea:

Welcome to the forum! Thank you for your post.

You can use input 1 for the left channel and input 2 for the right, but make sure both L1 channels have the same EQ and volume settings to maintain the balance.

Alternatively, you can connect from your keyboard's headphones output (or its dedicated mono output, if it has one) with a single cable.

Hope this helps!