Joining sheets of plywood perfectly flush is going to be nigh impossible. The veneer on most plywood is extremely thin, so you only have a few thousandths' tolerance if you hope to sand the joint smooth without sanding through one side of the veneer or the other. Biscuits are not going to help much.

The best thing you can do is make the joints look intentional instead of invisible, but it's hard to say how do that without knowing more about the context / project.

Use a piece of solid oak 1x2 or 1x3 between the pieces of plywood. That way every joint is plywood to solid wood. Set the biscuit joiner just a hair different for the solid wood, so there will be a little something to sand.

I am curious, how large are these glue-up panels? Never use scraps of plywood to make a larger panel that could be cut from a whole sheet. You will spend way more in terms of labor to achieve inferior results. Just go buy another sheet, it will save time and $

It was a last minute idea to avoid boring plywood sides on some book cases. There are relatively large center panels within a border, so it is like a frame&panel door with the frame and panels flush. It has a kind-of parquet look which I like. The construction is disappointing.

The narrower "frame" components could be solid wood. Alternately I could try a narrow shallow rabbit around the panel sections.

Thanks for the advice.