Would it be more economical to install central heating return pipe straight over feed pipe maybe insulating them together?

Returns are not insulated.

OK but if return to the boiler would be warmer wouldn't that mean boiler thus has to heat water for feed less? Returns are not required insulation but there's no harm in insulating them except to ones pocket is that not true? The idea is to install return straight over the feed and insulate them both together into one. Would that work all right with combi boiler, water pump etc would there be any disadvantages of any kind?

I was thinking air, we don't have boilers in Atlanta. Sorry, not a boiler person.

For what it is worth dept.(I am a little bored tonight)

Do you want the boiler heat to go to the return line or to the heaters(radiators)? You kind of are in a situation where if you exchanged heat from the hot line to the not so hot line, you rob that amount from the heat you would be sending to the radiator. I would be inclined to insulate each pipe separately.

Example:
Say you send 120 degree water out of the boiler and you give 20 degrees of that water to heat the return line, then you are only supplying 100 degree water to the radiators. This in turn them requires heat for a longer period because the water temp is lower at the radiator by 20 degrees. These numbers aren't correct, just an example to show you what you are trying to do. If you insulate both pipes individually more of the heat will get to where you want and you will lose less on the return side. Insulating the pipes individually should help regulate the heat in the areas that your pipes run through. I know you are trying to capture the heat off of the pipes to raise the return temperature so the boiler won't have to work quite as hard to bring it up to the desired temperature, but you are robbing Peter to pay Paul so to speak.