I mention one list box because you said that you want to be able to switch back and forth between the two listboxes. Whatever you decide to do in the end, in order to get values from a multiselect list box you will have to loop through the listbox using a Variant and store each of the records/selections in a string.
With the string you can adjust the other listboxes Rowsource
Here is some (untested) example code to get data from a multiselct listbox
Code:
dim varEmail as variant
dim strAnswer as string
if me.lstSelection.count = 0 then
exit sub
else
for each varEmail in me.lstSelection.itemselection
strAnswer = strAnswer & me.lstSelection.column (0, varEmail) & "; "
next varEmail
end if
debug.print strEmail