I have both the Genos and MODX 6, in fact apart from my Roland Integra 7 I sold my other keyboards and just kept these as they cover everything I need (with a few plugins as well on Logic Pro X). In terms of comparison, they both have superb sound quality. The Genos is my main board, just love the keybed and ease of use. I like to record covers of songs and finding staple instruments is just a pleasure on the Genos. The big touch screen, ease of layering voices, the sliders and OLED display, and the categorisation of sounds just makes it a pleasure to work with. In terms of sounds, the Genos excels in the acoustic guitar department over the MODX although you do get a bigger variety of variations on a theme with the MODX (and of course you can dive a lot deeper into the editing of the voice, DSP effects and so on. You can also layer more sounds with the MODX in a 'live' performance. The Genos has a few sounds which you don't get on the MODX including the Yamaha C7 grand, Kino strings, the super articulation instruments, acoustic stringed guitars including resonator, ukulele, steel strum, certain synth patches and pads, revo drums and probably a lot more. However the MODX excels in exotic synth textures, moving synth sounds with arpegiators, layered effects and so on. The MODX also has a bigger variety of string sections and ensembles, combinations of synth and real sounds, a good variety of electric guitars (different to those on Genos). Genos has a bigger variety of electronic and electric organs, whilst MODX has of course REAL FM sounds and can load Classic DX patches. Whilst the DX sounds on the Genos (including the great optional sound packs from Easysounds) are really nice and sound great, you can't beat the expression, dynamics and playability of the MODX real FM sounds. Both Genos and MODX can load new sounds and samples. I have a large collection of patches for both, the MODX offering the Bossendorfer pianos and some great analog patches although EasySounds also offer most of these for Genos now.
I find the Genos a great tool for building the framework or skeleton of a track, using the styles and it's onboard sequencer. I use Logic Pro X as my DAW and trying to set up the voice changes and sys ex commands for Genos is a bit of a nightmare, so I tend to record a skeleton with the sounds I want on the Genos sequencer and then import it into Logic. Recently I'm recording directly as audio though for individual sounds using the SPDIF digital audio output. I also love the joystick on the Genos, although for more expressive modulation control the traditional wheels of the MODX are better.
Before owning the MODX I had the Montage and prior to that a Yamaha Motif. I've always had a soft spot for Yamaha's acoustic and real instrument sounds, and I sold my Montage to get the Genos. However with the attractive price of the MODX and the fact it basically does everything the Montage does (in terms of sound quality and compatible patches) I just had to get the MODX6. It's an incredibly light keyboard which makes it great just to take into the lounge or even for performances elsewhere. My intention originally was to take the Genos to our church for performing, but even after a couple of years I'm still reluctant to take it out the house and don't have a case for it. I'm actually still using my Yamaha PSR-S950 for church which still sounds great.
So in terms of using the Genos and MODX together, they do share a lot of common ground in terms of the Yamaha voice setup. The MODX will give you more choice of piano sounds, and of course with its layering of voices enables a bit more realism than Genos, although I love the CFX and C7 pianos of the genos and the keyboard bed. You also don't get aftertouch on the MODX keybed, but it responds just fine over midi from the Genos keyboard. The live set feature of the MODX and its touch sensitive display also make it a relatively easy board to use, although the screen is considerably smaller than that of the genos and requires a slightly more nimble finger to use!
Hope that helps, I think both keyboards are unique in what they do and both have merits and drawbacks. Together though you shouldn't be wanting much more in the way of great sounds and variety of instruments.
Kind regards
Simon