Be careful. MPAN is not the only criteria for determining whether a site is singular or plural. It is just one of them.
If there is one owner with one postal address then it is likely to be treated as a single site - irrespective of how many phases are involved.
There is also a deadline of 18th October for the way that extensions to existing systems are treated (which this would be if they were to be accepted as a single site) - that's today.
And why would you want to register them with separate suppliers?
Not worried about being treated as same site, kind of assumed it would be.
Reason for different FiT supplier? Two reasons, 1. both supplies are with different suppliers (for historical (hysterical) reasons. & 2. I like to "play" the system and SSE are very slow with payments and I though I would try another FiT supplier to see if they were better. (all eggs in one basket etc.)
thanks
Roger