Two 5 cars is ludicrous with 2 extra cabs and 2 sets of catering taking up vast amounts of unused space plus the extra crew and no connection between the sets.
If it's the same 10 carriage train all day joined up on the same service, then - yes - a 9 car set makes more sense. But 2 x 5 off Paddington, divide at Westbury, rear all stations via Frome to Weymouth, front 5 Cary and Taunton, where it couples up to 5 cars from Birmingham and beyond and the new 2 x 5 carried on to Penzance - now that makes sense.