How would I go about creating a center line between these two closed curves?

Tween isn’t doing the trick on any settings I attempt.

Nope and it probably won’t… TweenCurves and the like only work reasonably well on relatively regular curves that have small changes in shape between them. There will also be curve sets where a “reasonable” solution is either ambiguous or cannot be found.

One attempted workaround is to create a line between the two curves to represent the first direction liaison, then use Sweep2Rails with the line as a section. While in the command, use “AddSlash” and start connecting the curves with as many slashes as you need to create a “reasonable” set of connectors between the curves. You will see the preview isocurves adjust as you add each slash. Once you are satisfied, complete the sweep and extract one or more isocurves from the resulting surface in the U direction to get your “tweens”.