depends on just a cpl of things really
Type of Traction
What Formation it is in
East/West type
Single or Double Set
If any are locked out which set
apart from that..easy lol
Probably on a normal weekday the only service that may cause an issue is the 15:07 Chester-Manchester for two reasons:
1. Different guards open the rear unit at different stations: sometimes it's Greenbank, sometimes Northwich, sometimes Knutsford.
2. Unless it's 150/2s and/or 156s it means that corridor connections can't be used.
Almost always if it's one unit being used out of two it's the rear one that's locked out of use and as there are no reversals on route it stays that way. Quite often if the rear unit is locked out of use the driver stops the train where he/she would normally stop it if there was no rear unit sometimes with part of the rear unit hanging off the back of the platform.
The only time I've been on a train where the front unit was locked out of use was when there was a problem with doors jamming on the front unit. However, I appreciate if that did happen it would cause a problem at Northwich in the Chester direction given that the driver can't go past the signal meaning the hump then can't be utilised.