“What I saw was…” vs “What I saw were…”
When you start with "What", you're referring to an incident or object that you've seen. So it seems more appropriate to use the pair what ... was like you were answering a question What was it that you saw? So say it like this:
What I saw was a driver and an attendant.
But you could indicate that you're referring to the people and say it like this:
The ones I saw were a driver and attendant.