Why would this be a problem?
Because the resulting hybrid concoction would require completely new gauging and testing, the costs and complexity of both would render it dead long before any real consideration.
If one wanted a more logical (though equally unlikely to occur) option regarding 350 carriages, create 6 coach 350s/450s in the spirit of 458/460.
5 (possibly 6) coach 360s could have been an interesting prospect balancing service capacity, with utilisation of St Pancras (MML). Though now even more dead with the deletion of fizzy knitting (and other work in the Leicester Area - Wigston Triangle to Syston Triangle)
For the most left field of West of England Mainline take, there is the following logic rabbit warren (regarded as genius/nonsense/insanity; please delete as appropriate):
The 444s and 185s have similar 23.xx metre carriage lengths.
The 444s have Driving Motors sandwiching 3 Trailers.
185 have 3 carriages with individual power units.
So conduct a historically inspired reusing of traction equipment through a below the solebar transplant from 185 to the 444 Trailer carriages.
This would create a DC + Diesel Hydrokinetic Multiple Unit which we shall call a 771 to be vaguely correct, 7xx a multi-mode multiple unit, 47x being absent (spare) from the 4xx series and 4x1 as they would be express units. (Class 666 would feel more appropriate).
They would have ratings of 2000hp on DC, or 2250hp on Diesel, or 4250hp if
One really wanted to escape Pizza Express in Woking.
Due to the added weight of 3 power packs the acceleration would be dented on DC, but acceleration isn't what the 444s are built for, demonstrated by being ~50% longer than a 450 with the same power.
Comparing our 771 to the current stalwarts of the West of England 'Main' Line (feels wrong calling it a 'Main' Line with single line stretches) the 159, (taking the more potent 1200hp 159/0).
On DC power/carriage is an equal 400hp, whilst on diesel the 771 is a 50hp boost. This should somewhat offset the higher overall mass, and would be a 100mph unit rather than 90mph.
This would increase the minimum train length (ignoring the 2-carriage 158s to be kind) by 75% from 68m to 119m.
The reduction from 750hp/carriage to 450hp/carriage mitigates the 'overpowered' nature of 158s when not climbing mountains.
Should minimise the amount of regauging required if the transplanted motive power fit within the envelope of a Driving Motor et scrabble tiles.
If the acceleration rate on DC is felt to be too great then Pizza Express in Woking mode could be used to compensate, whilst liberating fillings and perforating eardrums.
This would allow 159s to be cascaded onto other routes to strengthen/replace less Sprinters (those that are not a 158 or 159, which should already be razor blades).
The 'loss' of 444s from some Weymouth diagrams could be replaced with shoe equipped 350/2s as we know how much Portsmouth Direct Line passengers loved the 450s, lets not enable the passengers of Southampton, Bournemouth, Poole or Weymouth miss out on this 'much loved' experience.
Would remove the use of diesel over ~50.5 miles from Worting Jn.
A twin set would be 238 between London and Exeter which is decidedly comparable in length to a 236m 9 coach IET.
Should be more fuel efficient due to fewer engines (being optimistic that it is the one day all of the IETs engines are working).
Especially per/seat with twin 771 having around 10% higher seating capacity compared to a 9 carriage IET.
Unlike twin 5 carriage IETs twin 771 would have gangways between units enabling passengers to access working toilet facilities, find a seat, and be bothered by someone unaware trying to hide from a revenue team.