Is it a good idea to put 1/4" XPS under engineered floors?
You can find the answer to your question in the installation instructions for the engineered floor that you are going to use, and if there isn't data there, call up the manufacturer and ask.

My guess is that it will not be allowed; the XPS is likely to deform easily, which leads that portion of the floor to be unsupported. Flexing of the floor at the seams is a bad thing.