Can I ask why you are exporting power from A to B as it were, and how much load it will be ?  As others have noted, this is tricky, and you cannot really avoid linking the earths if the two sources in the same building are within simultaneous reach, so unless one load is double insulated and in a plastic box, or up and out of reach, you really will have to use a common earth, and then whatever they were originally they are both acting like PME, unless you deliberately  make them both TT - a solution not without its own issues.
A church on PME sounds a bit unusual, but maybe the done thing in a built up area - I'm used to village churches being TT or TN-S.

As it sounds like you calling the DNO area engineers office, or whatever that role is called these days, you need to ask them about the options - you don't want to link things in a way that the armour of your cable ends up carrying some of the neutral current for other buildings on the same transformer - which is the sort of thing that can happen when supplies are sharing a building and bonding.

The lightning thing is an additional complexity - there should be a lightning conductor with its own earthing arrangements, and those earths should link to the building earth near the supply origin, at one point only, again to avoid diverted current. Cases with in effect 2 supplies require specialist input, from someone who can look at the plans and go 'aah'

Mike.

Can I ask why you are exporting power from A to B as it were, and how much load it will be ?  As others have noted, this is tricky, and you cannot really avoid linking the earths if the two sources in the same building are within simultaneous reach, so unless one load is double insulated and in a plastic box, or up and out of reach, you really will have to use a common earth, and then whatever they were originally they are both acting like PME, unless you deliberately  make them both TT - a solution not without its own issues.
A church on PME sounds a bit unusual, but maybe the done thing in a built up area - I'm used to village churches being TT or TN-S.

As it sounds like you calling the DNO area engineers office, or whatever that role is called these days, you need to ask them about the options - you don't want to link things in a way that the armour of your cable ends up carrying some of the neutral current for other buildings on the same transformer - which is the sort of thing that can happen when supplies are sharing a building and bonding.

The lightning thing is an additional complexity - there should be a lightning conductor with its own earthing arrangements, and those earths should link to the building earth near the supply origin, at one point only, again to avoid diverted current. Cases with in effect 2 supplies require specialist input, from someone who can look at the plans and go 'aah'

Mike.