Not really a true representation of what happened.
What tended to happen was the boundaries were drawn at towns / cities so as to prevent the silly situation of buses 'meeting' in villages.
Midland Red ran into Northampton for many years from Rugby and indeed further when they ran an express from Birmingham.
United Counties of course ran a regular service into Leicester (they still do) which was Midland Red territory.
Further south Aylesbury - though it had a United Counties, later Luton & District garage was served in NBC days by UC / L&D, London Country from Watford & Hemel, Alder Valley from High Wycombe and South Midland from the Oxford direction.
It wasn't clear-cut, and I may be completely wrong in my understanding of 'Area Agreements' but all these services into Northampton were 'express' ones; I'm not sure how the agreement affected them. As well, I was really referring to the times when BMMO was part of the BET group, and UC, the Tilling group later THC, before they came under common ownership with the NBC.
The BMMO service from the Rugby direction was the BMMO X96, Shrewsbury - Northampton. Operated from the Shrewsbury end the first Westbound departure from Northampton was late morning, and the last arrival, mid afternoon, so of little practical value from Northampton. Curiously, the X96 ran into Derngate (UCOC -owned) but the other BMMO services terminated at The Mayorhold, then an out-of-the-way part of town.
The Northampton - Leicester service was a BMMO 'X' service only. When their privatisation successor, Midland Fox, started to be aggressive in other areas, UCOC introduced a Leicester service in retaliation - it was a comparatively recent (ca. 1986) innovation. Of course, there had been a UC London-Northampton-Leicester-Nottingham express service for many years, but my experience was that it was never used for intermediate (i.e. to/from village) travel at all, only town to town.
UCOC operated a Northampton - Birmingham express service as a result of taking over Allchin's in 1933; this later evolved into a Birmingham - Lowestoft express service jointly with BMMO and ECOC, before withering back.
The Aylesbury situation was the result of a quirk of history. Premier Omnibus Co operated routes in London, and when it was clear that these would be compulsorily purchased by LPTB, they bought the Aylesbury Omnibus Co. (which, as its name suggests, had pretty much a monopoly on services radiating from that town) in 1931. Possibly adopting London methods, the entire fleet was replaced by new, obtained on hire purchase; this didn't end well, and the company was then bought, possibly in distress, by Eastern National, in 1933. E.N. then resold most of the routes to the other operators surrounding Aylesbury, keeping some for themselves. Possibly this division was the result of some sort of area agreement, as it would have been straightforward for E.N. to simply continue to operate the whole network, though more cost-effectively.
But nothing is clear-cut, and bus history is not well documented. Whatever the reasons, and it seems odd that services from Northampton Westwards (towards BMMO territory) were always sparse, particularly considering how intense bus services were in other directions. Whether this is due to lack of demand, or potential demand was not satisfied because of bus company policies, isn't clear to me, why was Northampton to Banbury (market day and Saturday, 2 return trips) so poorly served compared to, say. Northampton - Harborough (reasonable service daily via 2 routes)?
As an aside, I recollect in a history of the SMJR (of which the Northampton and Banbury Railway was a constituent) that one of the best sources of income for the N&B (a low bar!) was the sale of hay. When times were slack, station staff set out with scythes in the summer and cut lineside grass on embankments and cuttings.