I'm not sure what this would achieve. The Sheffield & Nottingham trains are flighted together leaving St Pancras at XX02/32 to Sheffield and XX05/35 to Nottingham. So you can sort-of imagine that they are joined, just by a coupling that's about 3 minutes long!
As a reasonably regular traveller, the St Pancras issues are not - in my experience - due to enormous numbers of people (obviously Easter was exceptional)
1- that the two trains start boarding at around the same time - and when boarding starts, it's the ticket gate capacity that is the rate determining step
2- It's not possible to allow boarding to start too early because turnaround times are quite short
3- the area where people wait is quite constricted by the design of the station
4- People don't want to wait further away from the platforms (and I don't think the design of the station is particularly helpful here)
No cheap & easy solution to the issues at St Pancras, otherwise I reckon they'd have been done.
1: Absolutely agree. It is often
exactly the same time as well, which, given the departure times of the Corbys at xx:15/45, means every single person waiting outside the concourse then wants to get through. I have occasionally seen staff allow the wide-aisle gate on the 'exit' side be used to clear backlog.
2: While this isn't always the case there are some particular notable issues (e.g. the arrival at 1254 for the 1305 NOT departure - the arrival is often late and on the 'A' end of the platform as well)
Re. 4 I agree, I want to be waiting nearby so I can board immediately.
I do think though that there are often enormous numbers of people - when one of the cl 180s was running round with just 4 carriages, it was regularly full & standing in shoulder peaks 10 minutes prior to departure. When there are issues over the road at King's Cross, STP becomes absolutely overwhelmed.
The above suggestion about splitting at Leicester would likely yield little benefit (as mentioned above because of the flighting) - far better to run 10 cars from the get-go on as many known crowded services as possible during the week. Weekend travel experience now suggests that weekend services really need every (available) unit out where possible.