I am planning to build a car nose cone with a split mould using infusion. I plan to infuse each half, then bond these halves together (soric core)

have any images of the nose? 

If you use 2x200g cloth each side and a 3mm soric core you'll end up with a 4mm thick moulding.  This should give enough area to bond directly.  If you're worried you can then add some carbon tape down the inside to firm it up further.

The bonding thickness I'm not too worried about - It's trimming it accurately so it mates up properly - I can trim it in mould, but how to do it without damaging the tooling coat on the mould?!

Simples, very carefully!

surely a joggle joint will be best ?  than trying to join butt ends ?

If you use the excess flange material as a guide, you can get it very close to perfect without needing to trim it in your mould. Use an orbital sander with a 40 or 60 grit disc to sand away the excess material until it becomes so thing that it just breaks off. This is then the exact dimensions of your original part (provided you did a neat job of sanding it away).