I've always had the tagalong boat pull up to the back deck while the houseboat is idling in neutral and have someone ready on the back deck to hook the tow rope caribiner to the bow eye of the tagalong and hold the boat . Have the driver of the tagalong turn off the engine and trim up then he can get off the tagalong onto the back deck then the houseboat captain can shift into forward when told to by the back deck folks. Obviously have the tow rope on the back deck to feed out as the houseboat slowly moves forward. If you are nervous about the engines on the houseboat running while transferring the tagalong captain you could have the motors off but in neutral the props are not spinning which I'm sure you already know. 1/2" gold braid is pricey but super strong. I don't think I'd use polypropelene water ski tow rope. I guess I've been lucky, I've never had to make a harness. By the way, have the tow rope already cleated on the house boat. I've pulled as many as 3 power boats and 2 jet skis with no problems. Just have your most capable people doing the job.