I have 2 RCs suitable for my Mini 3: new one with a screen and RC-N1. However, I can have only one of it paired, and if I use another one it warns me and suggest to unpair another one. Why is this? Why can't I have several RCs paired to my Mini 3? DJI, please change this.

Reason.....Linking/pairing a new controller displaces the old controller. The drone ONLY accepts being linked to one controller at a time. ( I am aware that the latter is a rewording of you post but it is also part of the reason.)  
There are a few DJI drones that accept two controllers, either singly or simultaneously e.g. the M2P/Z, but one controller is designated as the primary and the other the secondary. The primary's sreen/phone has the greater authority to make menu changes and any commands issued by the primary take precedence. Even if the secondary is the only controller connected its screen/phone has very limited access to control menus.
Without such designation if the drone stored both linkings what would happen it both controllers were switched on and in range at the same moment in time.

Sean-bumble-bee Posted at 7-17 04:23
Reason.....Linking/pairing a new controller displaces the old controller. The drone ONLY accepts being linked to one controller at a time. ( I am aware that the latter is a rewording of you post but it is also part of the reason.)  
There are a few DJI drones that accept two controllers, either singly or simultaneously e.g. the M2P/Z, but one controller is designated as the primary and the other the secondary. The primary's sreen/phone has the greater authority to make menu changes and any commands issued by the primary take precedence. Even if the secondary is the only controller connected its screen/phone has very limited access to control menus.
Without such designation if the drone stored both linkings what would happen it both controllers were switched on and in range at the same moment in time.

I know that now it is as you described. Question is why is it this way? What prevent DJI to allow multiple RCs to be linked to the same drone? It is "pro" drone by both name and price, I'd expect to not have artificial restrictions. Are there any unsolvable technical issues with 2 RCs? I tend to think "no" because 2nd RC may be linked temporarily.

The answer probably lies in two questions and their answers which I would summarise as safety and cost.
1) What would happen if both controllers were switched on and trying to control the drone simultaneously? If there is no hierarchy what if one controller is commanding left and the other right? How many folks go flying with their kids in tow and have little hands going everywhere to copy Mummy or Daddy. Controllers and drones have been accidentally switched on in bags etc.

Yes there are other ways to avoid that as, from memory, is the situation with the FPV where two controllers can be linked but only one is ever allowed to be in control. From memory with both controllers on I had to switch the controlling controller off before the second controller would take command.
2) If allowing two controllers to linked to one controller requires additional hardware would you pay for it?

Besides it takes all of 15 seconds or less to swap a controller using the controller/drone button pushes. It's no big deal.
I have three mini 2's and 4 controllers and regularly swap them around, in fact in the last flying session I used the same controller and phone to sequentially fly all three mini 2's. I have done many more than that with a P3 as I wanted details from the controllers.

If you forget that you want this feature I am pretty sure its absence will soon cease to irritate you.

Sean-bumble-bee Posted at 7-17 07:41
The answer probably lies in two questions and their answers which I would summarise as safety and cost.
1) What would happen if both controllers were switched on and trying to control the drone simultaneously? If there is no hierarchy what if one controller is commanding left and the other right? How many folks go flying with their kids in tow and have little hands going everywhere to copy Mummy or Daddy. Controllers and drones have been accidentally switched on in bags etc.

oh, my bad - I was not clear. There are RC that is linked (i.e. registered to use with) drone, this is not the same as connected (have wireless link active). I don't want to use 2 RCs simultaneously (no 2 connected RCs). Instead I want to be able to use 2 RCs in any order. Say, 10 flights with RC1 then 10 flights with RC2. Today, if you want to use 2-nd RC more then 5 times you need to unlink 1-st one.  What I want is to have 2 RCs linked,, without need to link/unlink often. So no conflict, if one RC is currently connected to the drone, it will reject attempt to connect from 2-nd one.

Ah, would you be referring to the binding associated with the flyaway insurance?
"Binding" coming from DJI's use of the word "bound" in the Flyaway insurance documentation.
If not then I guess I am being thick lol.

If I am correct then it is my belief that the one to one controller-drone or controller-DJI_account binding is the foundation of the feature that grounds a drone in the event of a flyaway and use of the Flyaway insurance. I see you point that it is possibly just a software switch but unless many people request it then I doubt DJI will produce an update to facilitate this.

Thinking that this binding or the possiblity of binding is brought about by the throwing of a software switch was the basis ofhttps://forum.dji.com/forum.php?mod=viewthread&tid=268456

There are 2 threads over on mavic pilots that may be of interest
https://mavicpilots.com/threads/ ... 28528/#post-1453378
that contains a link to the second one.

what is your benefit of linking both than ?