Boot windows bios from second internal drive MBP Mid 2012 I'm facing a little big problem concerning my windows partition:
State of the art is, that I removed my CD-Drive and replaced it with a second SSD. Therefore I thought of moving windows 10 to one disk and leave os x on the "main drive" might be a good idea (just in case of possible future drive problems to fix them more easily). Although, Mac lists them both as internal drives, I firstly wasn't able to install it via Bootcamp on the second drive, so I installed windwos without Bootcamp directly on the drive using efi boot. Unfortunately efi boot isn't completely supported on this MBP model, (soundcard and TB2 for external displays don't work) but at least everything else worked that way!
This is why I turned the efi boot into a bios boot.
It worked perfectly, when only the drive with windows was inserted but when I added the second drive with os x, suddenly booting into Windows wasn't possible anymore! I did a little research and found out, that windows obviously needs to be on the "first" drive (disk0) to start. So, I shuffled the SSDs and was immediately able to boot windows again. Happy that I finally reached my goal I switched to os x. 
But when I tried to boot back into windows it didn't work again. 
I tried to recreate the boot files for windows with "bcdboot" and also created a startup partition on the os x drive of 200MB with the windows boot files but it's still not working.
So now I don't know what to do anymore! I don't want to shuffle (or unmount) the disks every time I want to use windows!
Is there a possibility to do a work around for that problem? (A virtual machine is no possible solution as I'm in need of full RAM capacity).
Thanks in advance!
Below is a excerpt From page 6 of the Boot Camp Installation &amp; Setup Guide (Copyright 2011). These instructions do change significantly with the next release of 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.


Back in 2011, all Macs were still BIOS booting Windows. Although the excerpt does not explicitly say Mac Pro's where the only models that could BIOS boot from a second internal drive, this was already generally known to be true. So, the answer to your question is your 2012 MBP can not natively BIOS boot from the second internal drive.

rEFInd is a Boot Manager. This means rEFInd does not actually boot operating systems. Instead, rEFInd saves information in the nvram. The Mac firmware reads this information to determine which operating system to load. The means rEFInd can to be directly used to BIOS boot windows from a second internal drive.

On the other hand, GRUB 2 is a boot loader. You may be able to BIOS boot GRUB on the primary drive and configure GRUB to boot Windows on the second internal drive.