One possible solution; I use twop roughly same sized
monitors and have set them so they are approximatey
centered horizontally. I can open Excel fullscreen or
nearly so and then open two files and size them to the two
monitors. On my system, when resized from full window or
minimized they return the last size and position used for
THAT  file. Saving a file in that size will cause it to
open that way unless another file is open in full window
mode.

Hope it helps