I don't think you can actually limit the pershing to 1.
I mean, if the game tells you can only have 1 pershing. Just decrew your tank and walla you have no pershing because your not in the tank anymore. Just call the second pershing and get back in your first one and there you go; 2 pershings.
Repeat process till your popcap is filled with tank crew men and man all your pershings.
Vehicle crew is disabled with Pershing.