Hi Clare .. Unfortunately I don't have any useful advice that I can offer yet as I'm in the process of toilet training my g/b twins too (2 1/2 years old). I think I missed a good opportunity a few months ago when they were showing signs of readiness, but nonetheless, we have been trying for the past couple of weeks. I have two potties available to them inside at all times, and then they also have access to the big toilet as well. It's been a lot of hit and miss, pardon the pun, but they'll happily do a wee/poo on the potty, and then next time, they've done it on the loungeroom floor or wherever they might be ! I do have books sitting by the potty which my girl loves to read while doing her business, whereas my boy likes me to sit and chat with him ! I've heard of some kids being trained within a few days, but as much as twins tend to encourage/compete with one another (monkey see, monkey do), we are definitely not going to be one of those "three days" stories unfortunately. I haven't been game enough yet to take them out to the shops/playgroup etc without a nappy on so I suppose this is sending mixed messages to them and maybe delaying their progress, but I'm not quite ready to do the whole toilet thing at the shops yet (or clear up a mess in the shopping aisle ... x 2) 
Good luck with it all, and I shall keep my eye on this post in case someone else can share some helpful tips.
Cheers
Denise