combining 2 separate sq waves inputs

I need a little help. My end goal is a sq wave with amplitude +-5V. With the PWM function and two power supplies (0-5V) and (-5-0V) I created two separate sq wave forms (0 to 5v) and (0 to -5V). How can I combine the two sq waves into a single sq wave that goes up +5v then drops to -5V? DO I need a comparator circuit? Feed them into a summing op amp? Please help

OK stop it now!

That is three posts all on the same topic. You haven't answered my question on this one:- http://arduino.cc/forum/index.php/topic,90071.msg681193.html#msg681193