I’ve been looking through the forums, but haven’t really found an answer to this question.
Has anyone been able to route the Midi In to the Midi Out (NOT Midi Thru!) in a Machinedrum?
If this isn’t possible, I’ll have to buy a midi merger 
No, that’s not how MIDI connections generally work on instruments.
What, specifically, are you wanting to accomplish?
I’d like to use my nord modular as a midi controller to the machinedrum midi in (and routed through to the monomachine to play in chords).
Then the machinedrum midi out to the monomachine midi in. And my monomachine out to all my other synths.
A little more info is needed.
Using a keyboard on the Machinedrum isn’t very usefull, its a drum machine, unless you mean for clock from the Nord.
You need to work out who’s going to be the master clock.
One day for a Gig (with only Monomachine and Machine Drum) I have connected the Mono midi out to the MD midi in, and then the MD midi thru to the Mono midi in.
And it works very well.
Then I try to put a Nord Lead in that setting, but the Nord lead goes crazy with unexpected sounds and blinking light.
So I think it will be very complicated to plug others synth than elektron with this kind of untraditional routing.
I think you need a midi merger.
I’m afraid I’ll have to go for the merger then.
Thanks all for the help!