install windows 10 via bootcamp not on the main drive
Below is a quote taken from the Boot Camp
Installation &amp; Setup Guide (copyright 2013).


  • If you have a Mac Pro with more than one internal disk and you want to install
  Boot Camp on a disk that isn’t in the first hard drive bay, remove the drives in the lower- numbered bays. You can reinstall the drives after you install Boot Camp.


So, it is possible to install Windows via Boot Camp, on a drive that is not the main drive. The maximum Windows that Apple officially supports is Windows 8.1 (64-Bit). Microsoft says you should be able to upgrade an existing Boot Camp Windows 8.1 installation to Windows 10.

Everything I have posted so far is by the book. However, you are proposing going off script by directly installing Windows 10. Officially, the answer is no, but unofficially the answer probably is yes. If you try, expect to encounter Scarecrows. In other words, expect error messages from Apple as some point during the installation that will say you can not proceed. There is usually are one or more ways to bypass any problems that you will encounter. You can try posting a problem here and see if anyone can suggest a solution.

Here is some advice.


Burn the Windows 10 ISO to a DVD. You may need a DL DVD.
Install a BIOS booting Windows.
Before installing, make sure the Widows Support Software has been copied to a FAT formatted flash drive. 
Select the Master Boot Record partitioning scheme when using the Disk Utility to erasing the flash drive.



  BTW, you do not need a license to try Windows 10. You can download and install without a Product Key. If you choose to keep Windows 10, then you will need to purchase a license.


There is always the possibility that Windows 10 is not compatible with your processor. I assume the Windows 10 installer would inform you, if this was the case. I have encountered PCs where I could install 64 bit Windows 7, 8 and 8.1, but not Windows 10. Basically, the processor did not meet the requirements given below. (This was taken from the Microsoft web site: Minimum hardware requirements, Section 3.1 Processor)


Compatible with the x86 or x64 instruction set.
Supports PAE, NX and SSE2.
Supports CMPXCHG16b, LAHF/SAHF, and PrefetchW for 64-bit OS
installation


On the other hand, I came across the Microsoft website: Windows Processor Requirements, which states:


  Intel Xeon processors are supported on Windows 10 Pro for Workstations and Windows 10 Enterprise only


Apparently, you can download and try Windows 10 Enterprise for free. See the Microsoft Evaluation Center website.

If you do not use the Boot Camp Assistant to install Widows, then you should follow the instructions provided by Microsoft. For BIOS booting Windows, the instructions for partitioning can be found at the BIOS/MBR-based hard drive partitions website.

I would recommend placing the System partition on the first (main) drive and then placing the Windows and Recovery partitions on a second internal drive. You could combine the Windows and Recovery partitions, if you wish. This System partition is only appropriately 100 MB in size, so the partition should have minimum impact on the space for macOS.


  Note: The Microsoft instructions do no include the Mac requirement that any drives that share macOS and a BIOS booting Windows must by Hybrid partitioned.