My current system (Goodman, 120,000 BTU) has a split return air duct with air feeding back into the furnace blower chamber from the left and right, passing through a filter on both sides. We are in the process for planning a replacement with a Carrier Infinity System and the contractor wants to remove the split and use only a single feed and one high efficiency filter.
Does anybody know what the advantages or disadvantages are of having a split return air duct into the furnace? Should I stick to the curent set-up?

It all depends upon the installation and how many CFM is needed by your system. If it were me I would want to keep the split deal since it supplies air to both sides of the furnace which insures that heat from the heat exchanger is removed in a even manner. It also might keep the system more quiet since the air is drawn in two spots instead of just one.

Probably they want to use just one return air because it will be more work to cut in two instead of one.

Keep your two return airs and if they argue ask them if they have done a manual D for duct size and a manual J for equipment size. If so get a copy of those and post here. NOTE I bet they have not done these to start with but it will make you sound like you know a little bit more than the average homeowner. LOL

Carrier will let you cut out one side of the furnace only for side return and that's opposite side of the blower motor mount.