How can I separate two panels bonded with urethane glue?
A saw, a chisel, or brute force. Cured polyurethane glue is largely impervious to solvents, and if there are any exceptions to that, they are probably absurdly hazardous to people.

If you don't actually want the small piece, setting up a router jig to plane it off the painting would be a relatively tidy/controlled way of removing it.