Duel Clienting would be fighting one client with the other.
Dual Clienting would be running two clients at the same time.

Variables are persistent in each tab of EUO. So, if you have a variable set to whatever you are hitting - %TARGET - then you were to use "uoxl swap" to change clients, that %TARGET is still whatever it was set to originally.
set %monsters AAA_BBB_CCC_DDD_EEE_
findItem %monsters G_15
if #findCnt > 1
gosub KillItWithFire
sub KillItWithFire
set %TARGET #findId
event macro 27 ; I think that's attack?
uoxl swap ; change to other client
event macro 27 ; Again, just attack last target
uoxl swap ; change back to primary client
return
Hope that helps