Have you had a look at this Twinsclub forum? They also list local clubs for multiples, where are you that you have no groups local to you? 
When my twins we around that age, it was probably one of the hardest periods because it's actually quite isolating being at home with very young children. I did go to a couple of toddler groups each week (inc. a local twins one, the only place where I felt truly comfortable and not a super-fecund freak, we've people knew to help by opening doors for the big pushchair and to keep an eye out for everyone's kids - you can tell I loved it, can't you
)
But mostly I just set myself little goals, feeding, a little bit of play, napping, a daily walk, a visit to a playground, feeding, napping, play and then bed. Small achievements!
The main thing that kept me sane was meeting other people (for me, other mums with twins) and I was fortunate to make friends with babies and see them on a regular basis (once a week? A fortnight?)
But I appreciate it is hard during the summer when many groups don't run, so alternatives may be library sing along groups? NCT or ante natal groups?
Could your health visitor put you in touch with other local families with similar aged kids? Do you have a toy library that you can use? We found that useful for many years.
I also used a nursery from age 2.5, mine did some sessions together and some apart. It was a great environnent for them and I think it really helped them. You may want to consider it as yours get older, they'll always be precious to you and growing up and learning to be apart from you won't change that. Good luck.