I have two circumference and a line connecting their centers. Here it is:

Now, using the 'Stroke style' dialog box I add a start and end markers to the line so the the line became a double arrow. Here it is:

Always in the 'Stroke style' dialog box there is the possibility to define an offset for the markers. Here I add some offset so that the arrows get outside the circumferences: 

Now my questions

1) is there the possibility to hide the part of the line inside the circumference? I would like to remove the red parts. Maybe I could set a fill color for the circumferences so that it cover the lines but.... isn't there another simpler solution?

2) Why if I change the offset of the left arrow it automatically change also the offset of the right arrow? Shouldn't they be independent?

Thanks!

I'd try creating the line using the diagram connectors tool (o).

I thought I had already suggested that: https://inkscape.org/forums/questions/connect-the-center-of-two-circumferences/#c41959

@Aero & @Polygon thanks!

Do you know what happen under the hood when you use connectors? I imagine the connector is a special type of bezier path.....

If you don´t want use connectors you can simply use to "arrows" as start and end markers of the straoke and let the points snap to the circle then. I guess no one will see/notice if it´s really 100% pointing to the midpoints.

Thank you @Polygon!