Hey guys,
so the situation is the following:
I have an encrypted Windows 10 running right now with UEFI. So far I disabled secure boot and the fast boot option in Windows.
Now I want to dual boot it with EndeavourOS which shall also be installed encrypted on the same drive. I was wondering, if I resize the partition to make space for EndeavourOS and just turn on encryption in the installer, will everything work fine?
I think that you’d need to decrypt your Windows hard drive prior to re-sizing it and then re-encrypt after.
Also, you may want to disable fast startup and hibernation. Both could cause issues while dual-booting.
I don’t have anything more to add along those lines. I dual boot Windows and EOS, but they’re on separate drives so don’t have experience with them on the same drive. I don’t have issues, dual booting on 2 drives, though. Windows shows up on my GRUB boot menu and I can choose to boot that or Windows. 
… I’m sure what you want to do is 100% possible, but I can’t offer any first-hand guidance on how to do it.
Another thing to consider: Not sure if you’re using BitLocker in Windows but if you are and you want easy access to your Windows files from there, maybe try something like VeraCrypt instead. VeraCrypt in Linux would then be able to mount your encrypted Windows volume so you could access files there. I really only boot into my Windows install for work and gaming but VeraCrypt has been great if I want to nab something from my Windows drive and don’t feel like rebooting and/or moving stuff around.