yeah that's not very hard, it would be easier for to show you if you posted a test map but i'll show you it. i think telling would be kindof pointless since you're so new, might not understand...
you need a timer variable, so don't forget to make one of those. the triggers use one. mine is called tmr.
it doesn't seem like there is a way to make the title of the timer window be longer, so my long timer window title is cut off. i suggest you use a smaller title than i used.
(Ctrl+B pulls up the variable screen)
edit: now that i think about it i forgot to add end of game announcements...
basically you want to put for the first actions of those triggers first of all turn off the victory and defeat triggers. second make announcements to the players like, player 12 lost his altar! game over! or, player 1 did not destroy player 12's altar! game over! Game - Display text to players "text here" and then put an action after that, wait 15 seconds. then you do the defeat and victory stuff. you can even put a 2 second wait after defeating one team before giving victory to the other. that might be good.