The 465/9 custom liveries can only be selected for the 51 4 car services, you can't select them for the 85 8 car services.
The game doesn't now how to apply custom liveries to double set units. If you select a custom livery say on the 375 or 377 it will only apply to the front set. I did notice yesterday that one of my custom liveries for the 395 appeared on an AI double set instead of just one set.
The 375 did have the same issue, in that the services were split into two liveries, so you could only use the custom liveries with one set of services. The last update combined all the services together so you can use the custom liveries on all services. I personally don't mind that only the front unit gets the custom livery. On the LGV route I have had some great combinations of liveries with the double units.
