Is there a way to control an android device with an external touchscreen? I am trying to solve something here.

I am trying to figure out a way to control an android device with an external touchscreen display. The android device would either be a Samsung Galaxy Tab A or a Galaxy Alpha. I have seen many connectors that allow you to display the screen from a tablet onto a tv or monitor but no easy way on controlling it using the larger screen (probably due to the video output on the tablet not supporting touch signals?). I have seen a few threads on various blogs that can just about manage this but they seem to be out of date and I can't see why this isn't easier.

Ideally, the external monitor needs to be larger than 20 inches and be a capacitive touch screen rather than a resistive type. I am using this as a control surface for music production and performance and sadly, the screen size on available tablets isn't big enough.

I would really appreciate any kind of input. I hope I'm not being a complete idiot and that this is really easy to do. Please let me know either way. Thanks

*Edit: Ideally, I would like to be able to use the tablet without the external screen as normal when I want. The extra screen size would be an added, optional extra.
If both devices are Samsung they have a brand specific app called SideSync which will allow you to link the two devices via a WiFi network. It's available via the play store or direct from Samsung. 
I currently have an S6 edge, a galaxy tab S and a Samsung Smart tv and can control the phone or tablet via the tv and the also the phone via the tv or tablet. 
Not too sure if you can control the tv or tablet via the phone as I've had no reason to try.
I have never come across an app which works in the same way if any of the devices aren't a Samsung but I've used this app with my current setup and with the old setup of S4, Tab 2 and older Samsung Smart tv I've owned over the last 12-18 months with no issues.
I hope this answers your question.