If, for some reason, I want to spend both ETH and ETC to the same address in a single transaction right now, how would I do that? The ether is currently in a pre DAO-fork address.
If you use an older wallet that doesn't support EIP155, your transaction may be replayed across the two blockchains. Are you interested in this out of curiosity or do you actually want to send ETH and ETC to the same address? If you actually want to make this transaction, would it be acceptable to send the same transaction on the two blockchains separately? Even if you use a pre-EIP155 wallet, there is no guarantee your transaction will make it onto both chains.