After creating a new blockchain, but before launching it, you can modify the necessary parameters in params.dat to make sure that the addresses in both chains are the same. These relevant parameters are:
address-pubkeyhash-version
address-scripthash-version
private-key-version
address-checksum-value
You may also find the multichain-util clone ... command useful for doing this, although it will copy across all the other blockchain parameters as well, including the default-network-port and default-rpc-port which you will definitely want to be different for different chains.