No need, simply get to the point where both tickets are at the "checkout" stage and you'll be good- as long as one of your sessions doesn't timeout. The airline can't not let the purchase go through if it's already about to be bought- as far as I know.
Shhh...I do it all the time!
Cant the price change untiul you actually make the purchase?
after you put it in your cart, they hold the seat for 15 - 20 minutes and then release it, if not purchased (I presume that's what Daniel means by you session timing out) I even did it once on the Aeroplan site (but I was using 2 separate FF accounts as well - I don't know if they'll allow you to log into the account twice)