Hi aacharya, thanks for your reply.

I can book it as all one ticket on Orbitz but the cost per person goes up to $3400 instead of about $1700.

The key questions seems to be whether either at MAN they would ticket the bags to GCM if I show them the second ticket or whether I could get the bags ticketed to GCM when recheck after customs at EWR (where I have the 3.5 hour layover). Does anyone have any experience of successfully negotiating that?