Zippy - you are more than welcome to come out for a spin in mine at some point.
I havent had a single engine rib of my own yet (getting one very soon tho!)... but with twin engines I feel safe because I know I have two completely seperate systems so if any point fails I have a good chance of being able to still go.
I can get on the plane with my boat on single engine, and I have recieved 19.2 knots on one engine... have to bring the other out of the water and slowly get up on the plane though.
If you did go two engines, there is no point in getting single fuel tank or single batteries - imho it defeats the purpose - like others have said in this thread - new engines are very reliable - it would be more likely a whole in a fuel line or dirt/water in the fuel(had this before!) ....etc... that it would be for the actual engine to pack up.
Two engines definately uses more fuel - I went down to christchurch last weekend, and it cost me about £100 in fuel.... single engine will probably do it with £60 (have you done that trip before richard?)
At low revs two engines give better manuerveribility in ways because you can put one in revserse and the other in forward and nearly turn on a sixpence!
The worst thing about two engines I believe is the maintenance... my recent "service" cost me about £300 per engine (I had some other bits done at the same time)
Overall I think if you are going to just run around the solent normally with other boats - I would suggest just getting one single bigger engine rather than two to give the same-ish horse power.
Give me a shout sometime and we can meet up and go for a spin