If I buy a second hand Mavic pro without a hand controller … can I link it to use my original controller ?
Yup, you just need to pair them 
Can one controller be simultaneously paired to two MPs?
I know one MP can be simultaneously paired to two RCs.
Don’t know… I only have one MP 
(but I do have two controllers)
Yeah. I know that works. Less sure about the other way around.
If you pair 2 rc to 1 mp they will be primary an secondary. The primary rc will all ways have priority. Good for newbie training flights😁
But in answer to the question. No an rc can only be paired with 1 mp at a time
That 1 RC to 1 MP was what I believed … and might be the issue for @opochka … without constantly re-pairing.
Yeah I tried to pair up both the ones I have so I can just switch out on flat battery. I can still do that but as I said one is set as primary and has priority over second rc 