Yes it can be done if the Touch Gamepad has the appropriate parameters in the Overlay.cfg file, in other words, literally mapping both User 1 and User 2 to the same mapping.
BUT you can do it without the Touch Gamepad by simply going to:
Main Menu -> Settings -> Input -> Maximum Users 2 -> RetroPad Binds -> Port 2 Controls -> Device Index -> #1
This will tell RetroArch: “Hey, there’s two players right now, but they are using one controller”
If you’re in mobile though… which I assume you are since “touch gamepad” unfortunately this “trick” is broken… the only simultaneously controllable inputs with one controller in Mobile RetroArch are Directional Inputs (D-PAD)… not sure if that’s by design, or an Android limit.