You can't do it directly.
A variable starting with _underscore is a local variable and applies only to the script in which it lives. A variable which does not start with an underscore is global and applies to all scripts (and everywhere else). Of course you can change the value of a variable as often as you like, just remember that it can only have one value at a time. If you want to "cancel" it use variable=null (I think).
In other words, to pass a value from one script to another use a global variable.
_tank = tank1
(Note that in MP it gets more complicated.)