Yes, if you buy Family Plus you'll get one 20kg check-in bag for the entire group. Plus a 10kg check-in bag for everyone (adult/child) on the booking.
Booking 2 separate reservations, both with Family Plus added, will indeed get you 2x 20kg check-in bags in total. However the surcharge (per person) you pay for Family Plus is higher on a 2pax reservation than it is on a 4pax reservation. So, depending on the extras pricing your particular flight (it's all dynamic pricing with Ryanair), it may be cheaper to simply add a extra 20kg bag while booking everyone under a single itinerary.

If you don't need the 10kg check-in bags it's very likely it would be cheaper still to not take Family Plus at all, and book only the cheapest fare adding only 2x20kg check-in bags.