check the comref for the exact command but i think the best to use is
count array or count trigger list
create a trigger for each side, lets name them
westplayers
eastplayers
have the trigger areas encompass the entire playing area, set them to repeatedly and anybody
in the condition fields have
(vehicle player in thislist) && (side player==WEST) && alive player
or
(vehicle player in thislist) && (side player==EAST) && alive player
then have a looping script or a script waiting with an @ command for something like the following command
@ (count list westplayers == 0) && (count list eastplayers == 0)
or maybe even an endgame trigger with the following condition line
(count list westplayers == 0) && (count list eastplayers == 0)
then have these execute your endmission stystem