I do this all the time, usually with a revenue reservation and a reward reservation, while I try to decide which to keep. Quite often what will sway my decision is what promo SPG is having at the time. Since my reservations are often made 6+ months out, I must wait to see what the promo will be.
Sometimes, I'll make 2 reservations at different hotels in the same city while I decide where I want to stay! I try to be courteous and cancel the extra reservation at least a month prior.