need advice on how to do a two tone finish

If you’re wanting the oak to be darker – have you considered fuming it with ammonia instead of staining it?
After fuming you can still do a little work, such as sanding flush, on the oak, without “cutting into” the darker colour.
Just an idea – I’m personally very fond of fumed oak, so I’m biased.

-- "Do not speak – unless it improves on silence."

You can stain the oak before assembly. Mask of any surfaces that will get glue and stain. Assemble everything and then clear coat the entire unit.

You could assemble, mask off the ply and stain, but I would be worried about getting some bleeding onto the ply, which you would probably not be able to sand out without blowing through the top ply.

-- John

Follow John’s advice and stain before assembly. I am completing a TV cabinet that I did this way. In fact I put on the top coats of Polyurethane too.
Michigander