Why does the l293 have a higher current than the l293D?
I agree that it is surprising that it is lower for the D version - the data sheet doesn't explain why although possibly it is because they use the same die and use some of the output transistor area for the diodes in the diode version - thus reducing the current capability.

The data sheet (TI L293 data sheet)shows that it is a bipolar part.

It is not usually desirable to parallel bipolar parts as they don;t share current well having a negative temperature coefficient.

The data sheet on page 8 shows where the diodes are - basically one diode fro each output to VCC2 and ground such that they only conduct when there is inductive kick from the load.