I have just had a new heating system installed. I have 3 rads which go to my conservatory with the last one on the loop in the utility room. The first rad (1) in the conservatory heats up fine has the thermo valve to the bottom pipe and the out valve to the top pipe. The next conservatory rad (2) has both valves to attached to the top pipe, the utility rad heats up fine is the last in the loop and is attached to both pipes.
The Rad (2) does not seem the heat up unless the utility rad is on full, I have spoke to the plumber who fitted it and he is coming back next week.
He has said on the phone that rad (2) attached to the return pipe with both valves is fine, but I think this must be wrong.
Re-piping the second radiator, so that it has a flow and return pipe will solve the problem.
Don't know why he's piped it up that way, it doesn't make any sense.
Having both tails connected to the flow, means you're getting the water that has been through the other two radiators first, and alot of the heat will be lost. Plus if you turn down the last radiator, your restricting the flow.
Don't ask - tell him to change the pipework. it's your house, and you paid for a system that works. Don't pay anything to have this work done.