Master/Slave setups ALWAYS degrade performance since IDE doesn't have any bandwidth sharing mechanisms. One drive working on something always completely blocks out the other one.
Put them on separate channels to solve this.
But no, the drives can very well run on different UDMA modes - if one is UDMA-133 and the other is UDMA-66, each will do its best when its turn comes. Only if you mix UDMA-66 (or higher) drives with UDMA-33 (or lower), you'll be kicked back to UDMA-33.