2405FPW PbP DVI & D-SUB not selectable - need help/advice

Hi,

i'm using a Dell 2405FPW monitor connected to a PC DVI and another PC D-Sub (VGA).
i'm loving to switch between DVI and D-Sub input to see the screen of one or the other PC.

sometimes i'd need to view both PC's on the same screen by using the PbP function (that's
the reason why i bought this monitor) but on the PbP menu i can only choose S-Video,
Composite and Component. D-SUB and DVI are greyed out and not selectable. also the advanced
menu function is not selectable.

does someone know what i'm doing wrong and what i have to do to see both PC's (DVI & D-SUB)
on the PbP screen?

thanks in advance for all answers.
frank