In popular dj software it’s possible to have 2 audio outputs, one for the speakers and one for the dj’s headphones, with different things playing. They do this with 5.1 sound front/back outputs or 2 soundcards etc. In Renoise you can loop a pattern and work on another one, but as far as I’m aware you can’t hear the latter. Being able to hear the pattern you’re editing whilst the other plays would be great for live stuff, and could have other uses.
Would it be better to have multiple master tracks, for example 1 for front speakers, and 1 for rear. This would provide reuired functionality if set-up correctly, and it also allows production of soundtracks/music in 5.1, which would be interesting.
You can already select any track to output to a different sound card port. You just can’t have more than one playhead position at a time.
(see http://tutorials.renoise.com/wiki/Mixer at the end of the third section called “Customizing the Master Layout”)
I don’t know if the OP is aware of that particular feature, but I’m pretty sure we were talking a pretty specific feature suggestion here - “Listen/edit the next pattern whilst current is playing”
Unfortunately, being able to play another pattern on top of the currently playing pattern is not easily done - it would require another playback engine to start alongside the current one. Could quite easily become something that would break a live performance, unless you managed to reduce the CPU consumption of every one of your songs by 50%.
The only way I could see this work, and being “safe” for a live performance, would be if the pattern was somehow rendered in advance. Heck, it could be even done with clever enough scripting - putting the “other” (rendered) pattern into a special, dedicated track which is then routed to a different output.
But, something perhaps be more obvious would be to reserve all pre-hear functionality (when loading samples, etc.) to a dedicated output. Also, the metronome could/should be routed to this special output, currently it just goes to the master track no matter what…
Yes, in this case all stuff is rendered pattern by pattern and then simply triggered as instrument in a specific track that is routed to a specific output. You can then route the master to your headset to process the patterns. Unfortunately, stuff like stopping pattern play or rendering to sample will cause any playing instruments to shut up, so there is no way you can let this all go by unnoticed in a live situation.
Even gets everything from Master track FX 
The absence of a feature like rendering, or even live-recording, while playing, right now, makes the effort of scripting this close to useless for a live situation.
There are some cool things to be done though, with the new Repeater (stutter) effect on the master. It does remember up to a full bar of music. Too bad you can not dry/wet it.
tldr; ask hitoritori how he combines multiple renoises. 
Renoise is more a composing/production platform as it is right now, than a proper dj tool.
Routing control would do the trick I think, thanks. The only problem would be accidentally pre-hearing samples on the master channel, but I guess if you had it all set up in advance you wouldn’t need to.
You can also set your prehear to monitor over the instead of the master track. Both the sample editor and the browser have a little inconspicuous dropdown menu with these options.
Not for mixing patterns, but mixing tracks within one single pattern including pre listening, there is a tool called “DJ Tools”. It is a simple DJ crossfade, if you have a soundcard with two outputs, you can also prelisten the selected tracks.
For mixing tracks of different patterns, but without prelistening, I have made a tool “Live Dive”.
Hmm, I got an idea… if you mirror the tracks of a pattern, and using live dive and the dj tool simultaniously, you may also have a prelistening effect.
Say your patterns a,b,c have each 8 tracks. Add 8 additionally tracks. Mirror the tracks 1-8 to tracks 9-16 in each pattern, so that each pattern has 16 tracks,
Set tracks 1-8 to a send track (master), 9-16 to a send track (prelisten).
Start livedive and do some changes in tracks 9-16; it should be heard only in the prelisten channel, not master.
If you like what you hear, do the same changes in tracks 1-8, so the master channel is affected.
Well this is not tested by me for now, but I will check it out later this week, if it is possible.