Hi,
I've ripped up the carpet in my basement and am preparing to lay engineered hardwood. I have 6 mil plastic vapor barrier to go down first. Then I have 19/32 OSB and 7/32 plywood. I had originally planned to just use the OSB and anchor it to the slab, but in order to not penetrate the vapor barrier I switched tactics to a 2 layer floating subfloor. In order to not return the OSB, I'm opting for a thin plywood underlayment sheet for the 2nd layer.

Which sheet should I do on top? I was thinking OSB on bottom and ply on top, but dont have a very good reason.

You are planning to fasten the two sheets together?
If you are using screws, or staples to hold the sheets together so your glue between the sheets can have better contact as it is drying, then you will get better holding power going through the thin sheet so the thicker sheet can be grabbed by the fastener.