Also to add re support teams - I do not think a support team of 1 is enough, I think it should be 2 minimum and 3 preferred. This will give the support teams enough time to have a rest at times as they need their wits about them as supporters just as much as you do competing.
I would also say that they need to be people you know and I don't mean on a superficial level. They need to know your race face; need to know how to handle you, to say the right thing, to know what's right for you. It's no good having a Sherpa who will argue with you or will get upset if you bite their head off because you're tired. Similarly you need to respect them and what they can see from the outside. Whilst you may feel strong enough to do another lap, for example, they may see tiredness in you and suggest a rest break. You have to be prepared to consider their opinion and go with it if necessary
To a certain extent they have to deal with everything so that you can just concentrate on the swim, bike, and run
Disclaimer - have never done an IM, let alone a Double, nor even fully supported someone taking part. These are my musings from having been there for the last two years and seen what some of the supporters have gone through.