even with congestion charge and central London parking? Also, you assume they have a car available...
The only potential split (and I've not checked it for cost) would involve Huntingdon, and even then only if one of you has a Network Railcard- but then that's not valid on Groupsave as far as I'm aware. Got Oyster cards? PAYG or season? If PAYG, then off-peak travel card returns may be cheaper than off-peak return plus two PAYG singles.
Or, if you've each got a card that can take having a £300 charge (not a debit, a sort of ghost-charge where no money is taken) against it then you could try the Bike Hire to get between Kings Cross and the Zoo.
(Peterborough not being in the Network Card zone is baffling. Surely wouldn't hurt the coffers that much to extend it, even if Ely-Peterborough was kept out and East Coast was excluded. After all, it's a heck of a lot closer to London than Exeter is, and it's the only station on the FCC network not in the zone)