I have found a solution. It may not be the most efficient, but it is simple enough. There is a an array which is created on read:
onready var ActiveTweens = 
This array is then called when the function needs to know whether the Tween node is going up or down. When the name of the Tween Node exists in the array, the function switches to fade in and deletes it from the array, and vice-versa much like the standard toggle function described above:
Use interpolate_property to fade in to desired volume
Use interpolate_property to fade out to -80.00 dB
This way, the variables are stored such that each button and tween can access their variables at any time (or the same time), but not in a way that would cause them to "cross streams" and interfere like they would with a single variable. It also will be somewhat useful to be able to print this array for debugging purposes later, now that I think of it. Thanks for the help, you set me on the right path it seems and I figured something out that works.