AH !
Thats a good one. You have to forgive me, I didn't see the point about Enterprise Servers. I have written a whitepaper about load balancing E1 Enterprise Servers - but it takes a little more know-how.
In effect, you're going to create a "fake" application server - my suggestion is "APPSVR". Then have your physical logic servers (LOGIC1 and LOGIC2) load balanced to the virtual IP (the F5) on the right port (with persistence of course !). Logic1 and Logic2 should be sharing the same servermap (since they should be the same TYPE of server).
To get this working in E1, you need to create a logic datasource called "APPSVR" and set up all your logic mappings in your environment to "APPSVR". Now, when a user runs a BSFN, it'll map to "APPSVR" and the F5 will load balance the request round-robin to one of the two Logic servers (LOGIC1 or LOGIC2) - and will remember your session with persistency.
ok - simple so far. But, launching UBE's is a little trickier. To load-balance the UBE's - you first need a shared PrintQueue between the two servers. To absolutely ensure that one server doesn't overwrite files from another server, you need to bump up the Next Number for the servers (since the NN field is 15 numeric length, I like to start one server at 100000000 and the next at, say, 500000000 - that way its a LOT of jobs before they wrap, but the job numbers are roughly the same length!)
ok - so now jobs are load balanced and are running on both logic servers. The nasty trick after that is to then come up with a solution for single threaded jobs. After all, having a single threaded queue in E1 on both machines results in the job theoretically being able to be launched twice !
I have a fantastic solution for this - but I don't give it away for free. The only free solution I'll tell you to do is to hard-map the single threaded jobs to one server (ie, no load balancing of the jobs).
Hope that all helps. There are a number of threads on here that talk about this in the past - and I'm just flinging words at the keyboard so I might have missed something (!)