Presumably P and Q will agree to share future profit and loss in these new proportions.But that has no effect on the taxation of the accrued gain in Blackacre of 0; that accrued gain should be shared equally between the partners because it was a return on their investments when their investments were equal.And if the amounts they contribute are unequal, they will have some arrangement to account for that difference which the taxing structure must digest.
When they are not equal, astute taxpayers can exploit the difference. Suppose, for example, that aggregate outside basis is higher than aggregate inside basis.
Nonetheless, there are transactions that can break the equality of aggregate inside and aggregate outside basis.
An election is provided by section 754 which ensures, by adjusting inside basis, that this equality is in fact always maintained so long as the election is made soon enough.
Consider first the simplest case in which an inside basis adjustment under §734(b) is mandated because a distribution of cash causes income recognition to the distribute.
For example, suppose that P and Q form the PQ partnership by contributing cash of 0 each.
It should not be difficult to figure out how to tax two individuals who contribute equal amounts of cash to start a joint business in which each will own a one-half interest.