I did my JOC with CRM Aviation (
Jet Orientation Course) as it was the cheapest around at the time. This consisted of a morning session talking about differences between Jets and Prop aircraft followed by a 4 hour Sim Session (2hrs as PF and 2hrs as PM).
At the telephone interview, when I was asked how many hours I did for my JOC I told them 4 and I was told they could not proceed with my application. I challenged this as it wasn't written down anywhere a) what exactly is a JOC course (it's unregulated!) and b) they didn't specify number of hours. The decision was overturned and I got through to the next stage.
Not sure this answers your question, but I'm an example of someone who did a 4 hour JOC and got through.
As for Multiflight, they don't provide JOC Courses and they don't have a formal relationship with Jet2.