Hi Samantha,
I don't have any advice on how to sit with one son in the bathroom while the other wonders around the house. But I can tell you how I trained DS, who has both receptive and expressive language problems as part of his autism. We used an ABA method of toilet training, which involved first schedule training and then gradually teaching DS to indicate when he has to go. This type of method works well for kids with autism, but I think it's good for other kids too.
We started by just having DS sit on the potty in the evening, either before his bath (if it was a bath night) or before getting into his pajamas. After a few days, he started going pee in the potty at this time. Then we knew that he understood what the potty was for and I started taking him to the potty every hour.
Since DS learns best visually and through written language, we used the book "You Can Go to the Potty" by Dr. Sears to help DS learn what he was supposed to do. We also wrote social stories about going potty and made them pretty explicit. Once DS understood how what we wanted and overcame his initial fear of the big toilet, he decided that he did not want to use little potty anymore and wanted to use the toilet. This made things much easier for me, because DS could use public bathrooms. DS has therapy or related activities every day and there was no way we could have stayed home for days to toilet train.
When it came time to teaching him to tell us that he had to go, we again took advantage of his love of the written word. I had little cards that said "I have to go potty" in every room and taught DS how to go get the card and read it to me when we needed to go. If your son uses PECS, you can use PECS cards the same way. Or if your son signs, you can teach him to use the sign for "toilet".
As for rewards, different things work for different kids. If you use ABA with your other son (I don't remember if you do or not), you probably know how to find reinforcers. Potty training rewards work in a very similar way. Rewards don't have to be food. You can use strickers or special activities too.
We started training last summer, a couple of months after DS turned 3. So it's been about a year since we started and I would say that we are still working on it. For the most part he IS potty trained and has very few accidents. But DS still usually goes on a schedule and has some difficulty with telling me when he has to go. He is getting better at this, but we are still working on it. Also he is not from potty independent due to his motor delays and inability to dress/undress himself.
I think the main thing is knowing when the child is ready and working with him at his own pace. Some kids train in a day, some kids take months.
I hope that helps.
Gena
DS, age 11 and always amazing
“Autistics are the ultimate square pegs, and the problem with pounding a square peg into a round hole is not that the hammering is hard work. It's that you're destroying the peg." - Paul Collins, Not Even Wrong