Hi wim, you can use the Transform > Rotate > Rotate Axis rotation command to rotate around any arbitrary axis line in 3D space.

At the start of that Rotate Axis command you will pick 2 points to define the rotation axis. For your case there you would want to get 2 points on a normal going perpendicular to both those lines. To do that you could either set your construction plane onto the base of those lines and then drag the x and y axes of the construction plane orientation picker onto those endpoints, or also a quick way to do it is to drag out 2 construction lines from the base point along each line, that will then make a "normal" straight snap line available for your desired direction.

See here for an example of the quick construction line way:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4665.3

Basically 2 construction lines that come out from the same base point will form a temporary construction plane and you can snap along the normal of that plane , you want to use that to place the axis line for rotate axis.

Then the next steps for rotate axis are to either enter in angle value or pick 2 reference points - you would want to pick 2 reference points, one on each of your line endpoints.

- Michael