You can select host by host if you want single or multiple monitors, so you can mix and match, it's just the multi monitor ones will launch outside the main window. If you were doing single monitor sessions they would all be housed in tabs. It will then open a remote desktop window that spans both, but that's going to be outside the management window in a separate RDP window.

RDM will allow you to select use multiple monitors and you can either select all, or pick which monitors you want to use which is nice. You are going to run into an issue though in that it's not going to span the screens and keep it inside the management window. RDM will let you span monitors and if you maximize an app, it will maximize to the one screen, not all the way across like you are experiencing now. You might want to try Devolutions Remote Desktop Manager Free. Upgrade to version 2.82 or later for a fixed version. It was however later picked up by the sysinternals team. It had a security vulnerability and Microsoft had no intention of fixing it as they deprecated the product.

Not sure how to fix that as I ditched RDCMAN well over a year ago.

Rdc manager 2.7