Dual Boot two Windows 10 Pro issues

Hello
I just build a new pc and I decided to go for a dual boot (one will be used for work: music production/video editing and the other for casual stuff, gaming etc)
each of the two Windows 10 is installed in it's own hard drive (two different SSD drives)
The first issue is that if I choose the second windows OS option from the boot menu the system will restart
I was informed about this from the tech guy that build the pc. He also said that he tried the legacy boot menu but the same problem exists
I suppose I can live with that, if there is no solution (if someone can think of something let me know)
But I also have another issue. I have bought two licence keys of course, but when I completed the registration for the 1st of my two windows, the second one became registered too! I tried changing the product key for the second one but as you might expect the key got changed for the 1st one as well
Please note that the two windows installations have different product IDs
Can this be resolved? Is it "right" to use two different installations with one key? I am not "whining" about buying a second license, I just don't want to run into any possible activation problems in the future

Here are my specs:
I7-7700K
Asus Maximus IX Code
Hyperx 32gb ram
MSI 1060 Gaming X 6G
Samsung SM961 (1st windows 10 boot option)
Samsung 850 evo pro (2nd windows 10 boot option)
other hard drives for storage (2xWD Black 1tb,1 toshiba 3TB)
PSU EVGA 850 G3
Cryorig H5 Universal CPU cooler

darkling said:

The first issue is that if I choose the second windows OS option from the boot menu the system will restart
I was informed about this from the tech guy that build the pc. He also said that he tried the legacy boot menu but the same problem exists
I suppose I can live with that, if there is no solution (if someone can think of something let me know)

That is normal It will boot the default OS, if you change it it will restart. You could set the default OS to the one you use most in msconfig

darkling said:

But I also have another issue. I have bought two licence keys of course, but when I completed the registration for the 1st of my two windows, the second one became registered too! I tried changing the product key for the second one but as you might expect the key got changed for the 1st one as well
Please note that the two windows installations have different product IDs
Can this be resolved? Is it "right" to use two different installations with one key? I am not "whining" about buying a second license, I just don't want to run into any possible activation problems in the future

I don't think you can do anything about that as the key is linked to the hardware. I have a disk that I boot on 2 different hardware (with 2 keys). When I swap between them I have to re-activate or wait for it to do it itself when it gets around to contacting MS servers.

Yours is the other way around - 2 Windows with 1 hardware. The key you enter links the hardware to the key on the MS servers. The EULA says you shouldn't run 2 instances on the same hardware:

2. Installation and Use Rights.
a. License. The software is licensed, not sold. Under this agreement, we grant you the right to install and run one instance of the software on your device (the licensed device)

I can't see how running 2 instances with separate keys can be achieved. If you were to install a 3rd version that would activate automatically as well as the hardware is already linked to your key.

Anyway you have bought 2 keys so you are complying (or trying to) even if it isn't possible to use them both.

What lx07 said +1. The restarting is normal when multi-booting a UEFI computer, and you tried to comply with the two license requirement, it's not your fault MS has changed their software making it impossible for you to attach a second product key to your second installation of Windows. The same situation exists with selling a computer with a retail license for Windows. You can remove Windows 10 from that computer and sell it and re-use your retail license on a new computer - but the old computer will still have it's digital license for Windows 10 stored on MS activation servers and the buyer can install their own Windows 10 on it and activate it for free with that digital license. But that isn't your problem, it is Microsoft's - you did all that you could to comply with the EULA.

Thank you guys for the help . My previous experiences with dual-boot was with 2000/XP and XP/7 where things were different.