Hi,
Thanks for your post.
First I would like to explain that one is client system and other is server system. Either in personal test, nor production environment, it is not a recommended configuration.
Compared to dual boot system, you may consider of Hyper-V virtualization technology, it is a built-in function in Windows Server TP 2016 (whether included in Windows 10 depends on system version).
Regarding the issue, is Windows Server 2016 setup as native boot (Boot to VHD)?
You can used BCDEdit /enum to see if Windows Server 2016 in the BCD store. If not, you can use BCDEdit /set {GUID} to add Windows Server 2016 to the boot menu.
Here's an article that walks through configuring native boot.
https://technet.microsoft.com/en-us/library/hh825691.aspx
Best Regards,
Alvin Wang
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