Can you short two output pins of a decoder (74LS154N)
Do not connect push-pull outputs together. 

If they are always the same, just use one of them. 

There is no advantage to shorting them together if you stay within the recommended fanout (the number of inputs an output can drive) and there could be bad side effects such as a brief shoot-through that causes a glitch if the paths through the decoder are not identical in delay time.