What should I assign as the inlet pipe orifice diameter for a hydropneumatic tank that has more than one inlet pipe?
The setup would depend a little on what is expected in terms of the flow through the two inlet pipes. If the flows through the inlet pipes are the expected to be the same, the diameter of the inlet orifice could be set to an equivalent diameter of the two pipes. The headloss in this case should be the same or similar to the headloss through the individual pipes.
However, if the flow through one pipe is much larger than the other, an alternative set up would be recommended. In such a case, you should instead set up the hydropneumatic tank orifice size or headloss coefficient such that there is no headloss. Then you would want then adjust the minor loss coefficient in the pipes to account for the loss through the opening between the pipe and the tank. This would give you an accurate headloss at the opening for a case where the flow in one pipe is greater than in another.
Modeling Reference - Hydropneumatic Tank