I believe you have not studied the wiring and do not know how the window controls work.
There is actually no problem with both relays being activated; this problem is already solved in respect of having the passenger press "up" while the driver presses "down".
All push switches (which includes each end of the rocker) are SPDT. Each one acts to lift one side of the motor connections from ground to the supply (or possibly the other way round) and all switches are "chained" so that those nearer the motor disconnect those further by their "NC" contact. If both "up" and "down" are simultaneously pressed, then both sides of the motor are pulled to the supply and it simply does nothing.
This page is not a bad illustration (about the best I could find in fact), and should suggest how you extend the presently installed system. Note that all switches or relays must be "break-before-make" action.