As long as I've been working with POL, gumps are still my nemesis

However, I see a clear problem here from what you've described.
The way you have your IFs defined, if someone selects both, they will get the 'red' item created, AND get the message saying they checked both. They shouldn't get the green item created, though.
My guess is what you really want is a radio button, not a check box... you want them to choose only ONE or the other, right? (in evidence for my gump ignorance, I ask this question: Do UO gumps HAVE radio buttons?

)
Assuming that's what you want, but you still want to use the check boxes, you need to re-order your code a bit.
Code:
if (choice[100] && choice[200])
SendSysMessage(who, "You chose both!");
elseif (choice[100])
Blah...
elseif (choice[200])
blah...
endif