I know when I found out I was expecting twins (at 27 weeks, talk about a shock) that I hadn't a clue what to buy. We kinda started with two car seats and worked our way up from there. By the time the boys were about 6-8 weeks old, they were either on the floor on a blanket, in their baskets, or in rocker chairs. Which was grand - until they started to focus on toys and look for a little more stimulation. I already had an activity gym but it wasn't big enough for two babies, and it took up a lot of space. Enter google, and enter the Tiny Love Gymini Total Playground. Or, the best €65 odd quid I've spent in a long time.
I bought this for just under £58 on amazon here, with free super saver delivery. It arrived via An Post within 4 days. It also comes with a plastic carry bag so you can fold it up and bring it with you if you're going to another house. I find it absolutely invaluable and we would genuinely be lost without it. It's even big enough for the boys when one is asleep and one still wants to play - there's loads of room to move one over and let him snooze away while the other lad has a great time kicking and grabbing.
Seriously, if you know anyone having twins, this would make such a fantastic present - I know it's expensive, but honest to god even if you clubbed together with someone else and got it, the parents would be forever in your debt. The boys are 15 weeks now and they spend ages cooing and gurgling away at the toys or at themselves in the mirror, I wish I had bought it sooner.
In my humble opinion, this is definitely an essential. It's safe, washable, roomy, stimulating, and everything can be taken off, moved around, or changed.
I'd have gone crazy weeks ago without it.