Tying H Bridge outputs
The 'frog' is that area in the centre of a set of railway points (British) or switch (North American) where the rails intersect. It presents a problem on DC model railways as it can short out the supply when the points are being switched.



Figure 1. Frog polarity control. Source: New Railway Modellers.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. Alternating polarity to each section of track using half H-bridge drivers. 'A' and 'B' are two halves of a H-bridge driving the upper and lower rails of Figure 1. The FROG connection drives the frog.

The switches of Figure 2 represent the transistors of a semiconductor bridge driver.


  In the system I am thinking of (to do with DCC Train control) there would never be a time when both tied outputs would be enabled but could be a time when neither was enabled. Is this possible? (or harmfull to the IC?)


If you disable the frog before switching the points and then re-enable it you should be fine. No damage will occur. It would be best to have a current limited supply to prevent damage to the H-bridges in the event of a short circuit which could happen due to other reasons anyway.

Some of your concerns are discussed in the New Railway Modellers link above.