I've got a feeling that we already will find out a lot history stuff that we don't know yet in the few remaining episode(s), so one onrail completely focused on being a prequel sounds reasonable. Two is a bit much however.
I think that's why there will be two prequels.
Prequel 1: This will revolve around all the family relationships that concern the MC. This doesn't just affect Lynette and her parents. We also don't know anything about Neil's parents. I also suspect that the MC has or had a sister or brother.
Prequel 2: This will directly affect the LIs and the MC, because I suspect that something might have happened to the MC and that EP1 is already a second beginning. It's strange that Derek fills the "Don't Drink Bottle" in EP11, but we already see this bottle in EP9 and can also give some to Sally.
What did Tommy say in EP9? We already saw that last year.