So it follows that the separate ends of what might otherwise be thought of as a crossover would have to be different point numbers if in adjacent interlockings. That must be preferred anyway to avoid castellation if a particular horizontal boundary is unavoidable.
Apologies, what I said about both ends of the crossover having to be in the same interlocking was rubbish. The two point motors can indeed be connected to separate interlockings. However, there can only be one "master" interlocking controlling the crossover - the other interlocking merely acts as a "message repeater" passing on the control commands out to "its" point machine and its detection information back to the master interlocking. So if the master interlocking fails, the repeater (or "slave") interlocking doesn't know which way its set of points are supposed to be. Or if the repeater interlocking fails, the master interlocking doesn't know which way the other end of the points are lying, or the state of the foul track-circuit over that end. So if
either interlocking fails, the other one can't clear signals over its point end.
On early SSI schemes (such as Paddington), at a horizontal boundary, the two point ends of a crossover were usually split between the two interlockings in the mistaken belief that this would improve reliability, when in fact it actually made it less reliable. On later schemes, we learnt to connect them both to the same interlocking. If that interlocking is running, trains can then keep running in that interlocking even if the other interlocking has failed.
Separately numbering the two-ends of the crossover to avoid a castellated boundary is not as useful as you may think. If one interlocking fails, you will still see the detection of the points in the interlocking that is still working. But if they are lying reverse, you won't be able to move them normal as you won't be getting the necessary locking conditions from the other, failed interlocking. And even if they are lying normal, you won't be able to set any routes over them normal, as such routes will require the other end set normal by the other, failed interlocking.