Invite her to join you.

"Why don't you join in?" You say.

"I'm not so sure." Katie replies.

"Come on, it will be fine." Shelly says.

"Ok fine, what should I do?" Katie replies.

"You can start by giving me a handjob and getting me hard again." You say.

She moves over to the couch and as Shelly takes her pussy off your cock, she puts her hand on it. Your cock is still wet from Shelly's juices and they lubricate the action as Katie pumps your cock back to hardness.