Anyone produced a GRP item that's finished on both sides?

So gel coat / matting / gel coat.

It is a case of making two moulds that fit inside each other with a gap of the required size.
Put on the gel and matting, then more gel and insert the second mould to allow the gel to cure.

Or is it simpler to make two items them bond them together.

Depends on the number that one is going to produce,

only a few - make both sides, bond together
If it is many then there are a number of ways that can be used,. resin transfer, solid core, foam core etc,

what did you have in mind?

As above if it's a one off part then I'd get/make two single faced sheets and bond them together.

I think I'd just go with your original thinking and use the first one sided panel made from the mould as the second mould side. To make clamping easy and reduce the risk of air bubbles, you could use a vacuum bag to apply pressure. May take a few goes to get the perfect panel.