Hey guys, if I create 2 partitions on a HDD and I install Hyper V on the first and ESXi on the second will I be able to choose from UEFI on startup which one to boot into?
Is there any reason why this wouldn’t work? 
I wouldn’t really refer to either of the two as operating systems, they’re hypervisors and thus differ from what one would normally call an operating system. Maybe nitpicking a bit…
You certainly won’t get it to work “out of the box” but if you are willing to fiddle around it should be possible to have them co-exist on the same physical drive. I can’t tell you how as I haven’t got a clue and I sort of doubt anyone has done it before.
If you want to try out both (for testing/comparison? idk…) I would suggest you use separate physical disks, it will make choosing between them easy and likely save you enough time to be worth the extra money.
if you make concessions for filesystems yes it is possible to dual boot with your idea. you will have to format the drive into at least 4 partitions possibly 6 . and use a filesystem that both can see ( preferable but not necessary all the time some exemptions can be made with windows but its finicky.) and it should be possible to install both O.S’s and just have to change some settings in grub or what ever boot loader you use.
side note i have done this with windows and a few other unix based distros but not with different low level hypervisors. your millage may vary as wendell likes to say.
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I would have to agree with @zpdr if you are going to try and set up two different operating software, do your self a favor and install them on two different disks, it is so much easier than trying to get them to work right on the same disk. I have tried both ways installing Linux and Windows 7 on the same drive and installing Windows 8.1 on one drive and Ubuntu on the other. I have found I have way less disk and file reads errors with each operating software on its separate SSD.
I consider them appliance level OS like pfsense or freenas but it’s still an OS 
Yes I totally agree, I just have a logistic problem with the number of HDDs I have available. I solved it though because ESXi can be installed on a usb stick. There is a best practice reference on VMware’s site on how to install the OS on a usb stick or memory card. https://kb.vmware.com/s/article/2004784
True it may be doable but I will not go down this road eventually, it’s too guerilla it for me 
One HDD for hyper-v and a 16 GB usb stick for ESXi is perfect. I can boot from bios to any 2 or to my windows disk and no fuss.