I'm a train guard and that is my understanding of it, ask your local station to unearth, and dust off a 'freely available' copy of the National Rail Conditions of Carriage for you, also available on the National Rail website at:  http://www.nationalrail.co.uk/system/galleries/download/misc/NRCOC.pdf

Please note this rule specifically excludes using a combination of two season tickets, unless the train calls at the station where one ticket stops and the other starts.

Blakey, we're more beep beep or ding ding up here! Only get the buzz buzz on some hand me down Wessex 158's!

Hey there folks, been watching this site for a while and decided to join up so I can add my two-penneth when I think I have something to add!

I'm a train guard from Edinburgh working for ScotRail, though I am a First Group sceptic and have not succumbed to the head office propaganda!

Normally interested in fare and customer service topics and will give you an honest insider's opinion!

I remember when I was a nipper (I can still remember that far back) on InterCity West Coast, constantly being reminded that I was entering first class by some smug troly dolly, to which I replied that I knew!
The first class child fare between BHI and BHM was less than the price of a cup of tea from the buffet so I was happy to pay it!

Back to the original post, it's unusual but possible for XC^▸ (Cross Country Trains (franchise)) to use their Turbostars on the Manchester - Birmingham route, using both ex VT^▸ (Virgin Trains - former franchises) and ex CT crew, expensive though.
Never heard of this happening Birmingham - Bournemouth but as far as I'm aware the 170's are passed for the route.

VT^▸ (Virgin Trains - former franchises) seem to be suffering from short term memory loss, it was only around twelve months ago that they were hailed along with Chiltern and Network Rail for coming up with 'Blockade Buster' diverted WCML service via Leamington and High Wycombe to reduce the need for replacement bus services. VT drivers were even trained on the route using Chiltern's Bubble Car and drivers! They do seem to have it in for DB^» (Deutsche Bahn - German State Railway - about) Schenker though, with the proposed and now shelved Shrewsbury service and introducing a morning service from Wrexham via Chester treading on WSMR^▸ (Wrexham, Shropshire and Marylebone Railway)'s toes and now trying to muscle in on Chiltern!

When using a combination of tickets, if one of the tickets is a season then the train doesn't have to stop at the boundary station.

Or, as National Rail put it:

19. Using a combination of tickets
You may use two or more tickets for one journey as long as together they cover
the entire journey and one of the following applies:
(a) they are both Zonal Tickets (unless special conditions prohibit their use);
(b) the train you are in calls at the station where you change from one ticket to
another; or
(c) one of the tickets is a Season Ticket (which for this purpose does not include Season
Tickets or travel passes issued on behalf of a passenger transport executive or
local authority) or a leisure travel pass, and the other ticket(s) is/are not.
You must comply with any restriction shown on the tickets relating to travel in the
trains of a particular Train Company or Train Companies (see Condition 10).
If you do not comply with this Condition, you will be treated as having joined the
train without a ticket and the relevant parts of Condition 2 or 4 will apply, either
to the entire journey, or from the last station where the train stopped at which at
least one of the tickets was valid.
For the purposes of this Condition, a ^leisure travel pass^ means any multi-journey
ticket (excluding Season Tickets) valid for:
(i) at least 7 consecutive days; or
(ii) at least 3 days in a period of at least 7 consecutive days
and includes rover tickets, travel passes, flexipass tickets and BritRail passes.