What you are asking is impossible in the pivot table itself as you are asking one part of the pivot table to aggregate via one method, and another to aggregate via another method, on the same column ..
It can be done using a calculated item. For your example above you'd use the formula ( $1 + $2) / 2.
.. But thats HIGHLY explicit, and would only work if you KNOW how many buildings will come back in the query.. EVERYTIME.
Why not restructure the query to combine it into a single query using an combinations of filters with OR in between?