Not sure what you mean by "multiple forms" (as you can submit only one form at a time). Are you looking for a way to filter the values inside an array that you have already filtered via XenForo_Input::ARRAY_SIMPLE ?
I'd guess that you're not naming the input correctly - it needs the "" at the end. This is PHP stuff though, not XF. PHP will only see one value from the list if submitted without the square brackets.