I dont think so. Whenever you couple two trains they still are seen seperated by the game, having their own departure times and so on. So the AI dispatcher wont give 2 trains a green signal.
Sadly not, until DTG introduce a free mode element allowing the choosing length of trains it wont work as they are not just 'carriages' but as 2bad4you says it identifies them as separate trains
I suppose you can do it only from a storage station and you can drive only until the next station (example Acton Yard-London Paddington). I remember I coupled 2 HST trains and I drove from deposite to Paddington. But, at Paddington, I can't start a new service (in Reading direction) with my double HST cause the traffic signal was fixed to red (for about 1 hour)