I just recently did a gig around 70 minutes with the A4 and the ESX.
I took the ESX as master though because it can store tempo-settings per pattern and I had some tempo/scale changes during the set so that worked out quite nicely. What I experienced though was, that the Clock of the ESX is faster than it can produce sound. So the ESX has some minor latency. It’s not that wild, but its always a bit late. It was only noticeable in one track so I just filtered some transients on the A4 and it worked out very well. At least the crowd didn’t mind at all 
Ad patterns: I guess it highly depends on your material how to organise the workflow on both machines. I used the ESX as drummer only and the patterns on the A4 were very minimal, little pattern changes mostly worked with mutes and performance mode. So I had only one project for the A4. If one would need to use multiple projects one should consider loading times. So it might be a good choice to use the ESX as a master so at least some music could keep running on. I can’t remember right now if the A4 will stop when loading projects. but I guess it won’t be that smooth. It’s easy to resync them when the ESX is the master, just hit the |< button on the first beat.
Anyway, so what I basically did, was to make heavy use of the ESX’s pattern sets. There’s plenty of storage for a full set. I used the ESX as a drummer only, so I made plenty of pattern variations and basically put 2-3 tracks on one pattern set. E.g Track 1 on Steps 1-8, Track 2 on Steps 9-16 etc. I just made sure to use the same entry points for tracks/transitions so new tracks always start at least on 1,5,9,13, that way I don’t need to write them down. (If a track has less variations than slots in the pattern set, I just put the last pattern a couple of times until the next 5,9,13)
I also used the Electribe Open Editor to give all the patterns the proper track name so I don’t get confused live (hate to use cheat sheets live, but it might help).
As for the A4 I did it in a similar way, organised patterns on a bank or two. I usually plan the track order in advance so on both machines it was from left to right. I did not use program changes or anything like that with just two machines it’s easy to switch in time, especially when using direct jump mode.
Well, that’s basically it. I had a great flow that evening. Actually the whole set was lots of improvisation as I only did the drum tracks on the ESX the night before (I usually play with a live drummer, but he got sick). Anyway, I think both machines play along quite easily. Hope my experiences could help a little.