Changing single select to Multi-select

[Promis_Jure]

Hello,

in a form, i ask people to select some items of a multiselect field.
them i would like to create a new field with the complementary -> all items except the selected ones).

do you know if there is a way to do that ?

[JeffJohnVol]

Did you ever figure this out?  I have a similar situation.