Danno,
If me.check1 = -1 and me.check2 =-1 then
"do something"
end if
If me.Check1 = 0 And Me.Check2 = -1 Then
"do something"
end if
As you can see, instead of using true use -1 and instead of using false use 0. Also, I would put the statements in the After Update event of the check boxes...