Miele has a thermal shield for putting two units together (same boxes as Thermador), but you don't need it, I think, if there's cabinetry between. There's also trim that fills the crack in front.

I know mine have something between them. You don't see it, but I remember having something in the order.

I don't see installation/heater kits available at thermador.com

I would call Thermador and see if they have installation kit for your model numbers and for the installation you are planning.

Installation of these units is not for the amateur. I'm an engineer who has GC'd multiple kitchens but found a pro to install these units, especially side by side. It isn't worth the agro to do it as they are very complicated.
Ask your experienced installer what is needed.

I finally got ahold of the dealer where I purchased the units from, and they informed me that there used to be some sort of heat shield sold separately, but now it is built into the units. Thermador does sell a trim kit to get rid it the line in the middle, but I was told it wasn't necessary.

That makes total sense - I hope they're right!

Me, too!