The principle difference between a double table machine and a pallet changing machine is in the setup:
A double table machine includes two tables sharing the same linear bedways resulting in a setup area in the front of the portal and one in the rear of the portal separated by automatic doors. The advantage to this system is less cost and the tables can be linked together to handle much longer parts.
A pallet changing machine shuttles pallets into the working area of the machine and can incorporate two or more pallets. The advantage to the pallet changing machine is the option for many more pallets as well as combining multiple machines into one system.