You could put the Drums on one drive.
With the RealTracks you should be able to symbolic link half or a 3rd to another drive. If you could get something to automate the creation of the symlinks rather than having to manually create each one.
Here's some links you might find some ideas, if you can do it by right clicking all the selected folders or a script rather than having to use the terminal would be better.

http://hints.macworld.com/article.php?story=20100502015718443
https://www.macupdate.com/app/mac/10433/symboliclinker
http://hints.macworld.com/article.php?story=20090408073633559

these were from the Loop Folder post as there are so many different ones that work on different Mac versions
http://mac.softpedia.com/get/Utilities/SymLinker.shtml
http://mac.softpedia.com/get/Utilities/Symbolic-Linker.shtml
https://www.macupdate.com/app/mac/41493/symlinker
https://www.macupdate.com/app/mac/10433/symboliclinker
https://www.bresink.com/osx/0TinkerTool/download.php
https://www.bresink.com/osx/300321023/Docs-en/pgs/0120-Files.html