This really useful function was dropped in the move to L4. Applying it meant that when I was working in a version of the English New Testament I could have my Greek text open and linked to that. If I changed from New Testament to Old Testament in the English, then the parallel Original Language text switched to Hebrew. This was pretty neat. Returning to the NT the OL text returned to Greek.
Yes, I know there is a workaround, but it's cumbersome. L4 was meant to be about improving and enhancing the Bible Study experience, but dropping Serial Associations was a retrograde step.
Bring it back asap. Like yesterday!