Where is the XT receiving MTC from or does it generate it's own? If it's slaving from another device, then slave the KeyStep to that same device (if it also sends MIDI tempo). Try turning off MTC on your drum machine. If you can't do that, good chance that i's not what's causing the issue. Not many devices actually run on MTC, usually only high-end sequencers, video editing suites and lighting rig controller consoles.
MTC is an actual time code ([h:m:s:frames]) and usually used with syncing to video whereas MIDI tempo is a tempo message telling a device what BPM to run at. I've never come across MIDI tempo combined with MTC doubling the tempo in the past, only when combining two MIDI tempo sources. Not exactly sure what MIDI RTM tells a slaved device to do tempo wise, I imagine it sends a BPM value every 24PPQN. I know if you somehow get the tempo from one source combined/merged with itself (usually by looping around a slaved device echoing MIDI Real-Time Messages (RTM) into a merge utility eg your MOTU Express XT) it will certainly add-up those tempos.
(I just look at the Wkipedia explanation and it describes a hex value being used as a "clock", although I really do think that "clock" is a misnomer considering no clock is ever involved as is in MTC.)
If you are sending the KeyStep to the XT then splitting-off to another instrument that then has a soft-THRU on it's MIDI Out connected to the XT, there is most likely your culprit.
