Hi Pip, welcome to BB and congrats on your twins!!!
For the summer baby thing, they will likely be in onesies or even just singlets and nappies a lot. You don't need to go crazy buying little clothes. Having said that, if you are having twins, they will likely be a bit smaller (I know it isn't always the case, but I'm sure it is usually) so I think it'd be safe to have some 0000 clothes. My Natty was in 0000 for a couple of months, and she was just a singleton. (But lived in singlets and nappies unless we were going out - she was born in December.)