Complex sentence: Older video mixers had two equivalent buses (called the A and B bus; such a mixer is known as an "A/B mixer"), and one of these buses could be selected as the main out (or "program") bus.
Split into simpler sentences: Older video mixers had two equivalent buses (called the A and B bus; such a mixer is known as an "A/B mixer"). One of these buses could be selected as the main out (or "program") bus.