Hey Jonas,
I’m struggling to understand what you hope to get out of this setup. Why not get the S4Mk2 that actually not only has 4 decks but also more complete controls over Traktor? – When you go and sell your S2 this solution would maybe be at lower costs, just saying. Also, do you want to plug your headphones from one S2 to the other when cueing the tracks? …
I think I somewhere once saw that you can’t actually hook up two controller to one laptop and use both of them with one software. Maybe I’m wrong with that and I highly recommend you doublecheck this if you are really willing to do this kind of setup.
The use of an external FX-unit as the RMX-500 is one really screams after a mixer with a dedicated Send-Return-Loop which are actually not that cheap if you are willing to get something with proper sound- and buildquality. You could of course simply connect it to a mixers master-out but you are then sacrificing the possibility of hearing & checking the effect before the audience can. If that doesnt matter to you, something small with 2 channels would be enough for this setup. For example, a Pioneer DJM-250 would do its job. – But yet again, if you factor in the costs of a new DJM-250 and a new S2MK2 you could easily be near the price of a S4Mk2 … But in general a 4-channel mixer would be 2 channels too much, that you could of course use to plug your phone / ipod / etc in just in case something happens. This would add even another layer to this complex setup. Also have in mind if you are playing out you need to transport all that stuff if you want to use it all the time, and may it be personal preferences, but I really wouldn’t want to carry a laptop (+psu), 2 controllers, a RMX500, a 2ch/4ch mixer and all the cables to go with this madness, would you? 😀