Sabre Dog, yes. It used to happen on occasions. As has been said, doing 2v2 against your own type wasn't/isn't the preferred option, but we'd do it if we had to for (for example) someone's ACL (Air Combat Leader) work-up or similar, pre-deployment prep, various other things. Or even just for stats or currency. 1v1, 1v2, similar and dissimilar ACT was part of our annual training syllabus, so, yes, we did do that. You may not have seen a lot of it because it was normally out over the North Sea.
Another possibility is that it was part of an exercise. Perhaps F-3s escorting a bomber package that had got tied up with the defenders. Lots of possibilities.
Courtney