It does not matter if it is steam, diesel or electric - If the engines are not equipped for multiple unit or two consists are not connected they are "double heading".

Each engineer controls their own engine/consist (independent) brakes, only the leading unit has its automatic brakes cut in and controls the train brakes.

Back in the old days of steam on long trains with a helper, the other engineer might have to help release and recharge the brake pipe during a release, but would have to lap their brake valve to allow the lead engineer to make a brake pipe reduction. This was part of the reason for the whistle signals to apply and release brakes. Modern brake valves have far hiher capacity than the old feed valves, and air is supplied from all locomotives throuh the main reservoir equalizing mu line. The old practice of having more than one brake valve controlling the brake pipe is outdated and outlawed.