Can I wire the two sides of a L293D Dual H-Bridge together if I only need one H-Bridge?
Yes, you can definitely parallel the two outputs of an L293D. I made a few stepper drivers based on L293D with parallel outputs and had no problems.

As this application note from ST Microelectronics (APPLICATIONS OF MONOLITHIC BRIDGE DRIVERS) states:


  Higher output currents can be obtained by paralleling
  the outputs of both bridges. For example, the
  outputs of an L298N can be connected in parallel to
  make a single 3.5 A bridge. To ensure that the current
  is fairly divided between the bridges they must
  be connected as shown in figure 2. In other words,
  channel one should be paralleled with channel four
  and channel two paralleled with channel three.
  Apart from this rule the connection is very straightforward
  - the inputs, enables, outputs and emitters
  are simply connected together.
  The outputs of an L293 or L293E can also be paralleled
  - in this case too, channel 1 must be paralleled
  with channel 4 and channel 2 with channel 3.




But you should be aware that the total current capability of the parallel outputs would be less than the sum of the two channels (&lt; 1200 mA).

EDIT: The only differences between the L293/L293E and the L293D are that:


the L239D has internal clamp diodes included
the L293/L293E has higher output current capabilities.