If the first drive is set to CS, then the second one should be too--that's cable select, and if it works with a single drive it'll work with a pair. Older G4s didn't support that, so you had to go master/slave, but that's apparently not the case with the newest ones.
I have a MDD Power Mac, and have installed an additional optical drive. Set it to CS like Makosuke says. All IDE connections on the MDD Power Macs use CS jumper positions (optical drives and hard drives).