Say coins in adress x move on gavincoin fork. Same transaction could be broadcast on MPcoin fork by anyone and vice versa.
In that scenario:
If the utxo for x comes from an MPcoin block
before the fork then the tx is
discarded as a double-spend.
If the utxo for x comes from an MPcoin block
after the fork then the tx is
discarded due to unrecognized inputs.