If there is a height difference between the two tow vehicles, you will probably want to get two hitch heads, each specific for the tow vehicle. The trailer height will never change, so the hitch head height will need to be adjusted (up or down) with a rise or fall in the shank so it's always level to the trailer hitch. Otherwise, if you use just the one, you'll need to make the adjustment every time you switch back and forth.
I think in time, you will probably end up towing with only one vehicle as the primary tow vehicle, and use the second one as a back-up only in case the primary vehicle is down for repair. If this is the case, then a second purchase is probably not necessary. However, if you are sharing the trailer with someone else and thus, the reason for 2 vehicles, then definitely get the correct height hitch head for each vehicle.
It's really your call, it's just a matter of convenience to have both so you don't have to unbolt everything and adjust every time you switch.
As for me, if I had 2 different tow vehicles .... yes... I'd get 2 hitch heads if the height was different between them. I have a utility trailer, and a specific hitch I use for it, as it's a fairly long drop-down. I have no problem getting the right tool for the right job, even if it's used only 1 time.