Is there an easy way to pull out a guide line and place it exactly between 2 existing guide lines?

You can double click on both existing guides and note their coordinates.

Case 1 :  they are horizontal. Note their y value and drag a new guide from top ruler set its y value to the average of these two value. 

Case 2 ; vertical guides : you already understood you have to set x values.

Another method is to draw a rectangle between the two guides using snapping options and drag another guide to the middle if an edge (always with snapping options).

Thanks

A variation that works will all orientation for the guides (obliques), provided the two first guides are parallel :

• Enable snapping to
  • guides 
  • nodes > cusp nodes
  • nodes > line midpoints.
• Use bezier tool to draw a simple segment : put first node on one guide, hold control to draw an horizontal or vertical segment and put the end point on the other guide.
• Double click on one guide aud duplicate, drag the duplicate on line midpoint.