Not sure 2 pitches is a group, but Nantcol is still going and was excellent when we visited, until the Welsh rains came for three days solid and we had to abandon the trip.
sounds like your just 2 families? what about shell island harlech? just book in as 1 family and then another booking for the second family, arrive at reception at the same time, just dont talk to each other until you get thr the gates then your left to find your own pitches so you can then be close to each other.
The mention of 'group booking' to many camp sites will stop you in your tracks! Often in T&Cs 'No Groups'! Also even bookings for single sex non-family groups on a single pitch is un-acceptable to many!
Depending on your group make up (single sex, individuals vs couples, ages etc.), probably best to play down the 'group' aspect and book two pitches individually. Some sites will allocate pitches on booking/arrival, so you may not end up side by side, whilst other sites will let you pitch where you want.
Pretty much comes down to select a likely site, then read their T&Cs to see what booking restrictions are in place. Being less than honest at booking, may get you on site, but risks being asked to leave if/when they realise you've breached T&Cs!
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.