I agree with the others. Make sure you separate the cats for a few days. While one is out and about, keep the other in a room with a litterbox and food and water. Then lock up the one who was out and let the locked up kitty explore. This, along with the rubbing with towels, will allow them to get used to each others' scents and to become curious about each other without the opportunity for aggression. After about a week or so of this let them both out at the same time and immediately give them a really special yummy treat (like tuna water or tuna). Feed them this treat at the same time at opposite ends of the same room. This way they learn to associate the other cat with great things (tuna). Do the tuna treat several times each day, moving the bowls a little closer to each other each time until they are able to eat near each other.
Bravo to you for taking in this poor kitty! I wish you luck!