A valve controlled by a guarded and (tell-tale) wire-locked switch in the cockpit would do the job. We had such an arrangement in the S61N and some S76B's
AW has some history with Bell as we all know......they only need to have a look at how the 212 was set up to solve this problem.
As Geoff suggests, an interconnect valve between the tanks to essentially turn "2" tanks into one is the easiest fix.