I think about connecting two central devices (nRF's 52840) using one common antenna. One device will be only RX and will scan for a advertisements, second will manage multiple connected perypherials. Is it possible to both devices use only one external antenna? Do I have stop scanning, when the second device is going to communicate with peripherials? I don't want devices to collide with themselves.
The same issues as when combining any two (or more) radio devices onto a single antenna would apply.
You'd need a diplexer.
As i know, diplexer combines two different signals from two antennae (two differents frequencies) into one signal. What I need is one antenna used by two devices with the same frequency (2.4GHz). Is it possible?
Yes, it is possible but diplexer might not be the ideal choice. Some alternatives:
Even if both devices can use the antenna at the same time with the circulator/power splitter you also need to consider the isolation - the signal transmitted by one device will arrive at the other probably with enough strength that it will jam any signal coming in from the antenna. The transmitted signal from one device will also travel 'along' the PCB and be picked up at the other, this might also be strong enough to jam unless you add e.g. a shield can or something around one of them or both.
Jamming is one challenge you will have to solve, if both radios are active at the same time you might also run into intermodulation between the radios, which might cause excessive spurious emissions. For this I would expect the switch alternative to be better as it would attenuate signals both directions. More attenuation could be added, it is possible to add 'directive attenuation' by using a circulator (isolator), but at this point, the entire system is starting to get quite expensive and complicated.
If you implement some sort of coexistence interface such that only one is active at a time, you would not have this problem and also not have to test for it, but then I suppose it would not make much sense to have two nRF52s.