Was just cruising thru the forums and saw your post. I'd wait until your Ds moves in, unless he obviously wishes to move forward. Since you are his babysitter he may be perfectly comfortable potty training during the transition. He may regress a little after he moves in, just with the change, but he may not. I'd just play it by ear.
Also kids don't work on two developmental areas at the same time. They work on gross motor (walking, jumping, running) and then they may pause with what they've accomplished in that area and move forward with another area, such as speech. They usually go back and forth between developmental accomplishments. It's pretty interesting to watch, too! LOL
Hope your transition has gone well and your Ds is doing great.
I popped into this forum because I'm going to be starting in with potty training our youngest soon. (As soon as she's over her cold.) Our second youngest isn't ready, so I thought I'd work with the youngest so she can teach her older brother.
I think he'll get the hang of it better if she is a trail blazer for him. LOL