Property Agreement

If we decide that we want to separate, but sell our home first, can we do a property agreement now?

A separation agreement needs to be entered contemporaneously with separation or after the separation. It really depends on how long between entering the agreement and actual separation is. You should try to come to mutual agreement about all the terms of the separation agreement, have it drafted, and enter the agreement once you are under contract for the sale of the residence and can specify what the date of separation will be.