I've got a similar (ish) setup: dumb controller; master keyboard with onboard sounds;
sound module. FWIW, mysetup is:
Controller's MIDI out -> Master 'board MIDI
in. Master 'board MIDI out -> module. AND make sure the master 'board has MIDI merge
enabled, that way all MIDI from the controller AND the master will end up at the module.
The additional thing you might need to do is turn the Local of the M1 off, so that when
you play the keys on that, it doesn't trigger its own internal sounds.
Then,
you just need to set your patches on discrete channels (1 to 16) assuming you want them
distinct; OR on the same channel if you want to layer them.
At that point,
can't help much more 'til know what you want to do. But what that setup should give you
is ability to play 1 patch on the 1080 from the M1 and another (different) patch on the
1080 from the K49, AND whatever the local patch is on the M1 (with the same notes
obviously).
Re-reading your post "the M1 plays both its own and the 1080's
sounds" - sounds like you do have the "Local" setting on the M1 on. If you don't want the
M1 to play its own sounds, but only the 1080's, you'll have to turn "Local" off. You can
then play the M1 sounds from the K49 (if you match the MIDI transmit from the K49 and the
MIDI receive on the M1) and play the M1 sounds from the K49 (as well as the 1080 sounds
etc etc.)
And if that hasn't confused you...
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Hammond M162, XM1. Yahama CP70B, PF2000, P200, P50m, QY100. Roland RD1000, JV1010. Alesis SR16.