Might be possible through hex-editing .dll files...but you'd have to ask in the Coding Forum to get a better answer than that.You can also make one or the other of the summoned minions be "unkillable" in monstats.txt (set 0 in the killable column if I recall...something like that). That way, when you summon the other type, the first type cannot die...so it won't vanish.
But an invulnerable minion usually takes the fun out of the game...go with the .dll editing option if its possible and you have the skills...and check out the Coding Forum if you need help.
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- JEFF
"A hero is no braver than anyone else. He is only brave five minutes longer."
For my latest mod (for LoD/expansion), download Version 1.0 of Sanctuary in Chaos - Or for old stuff (for classic D2), visit my orignal mod site at www.planetdiablo.com/sanctuary