Skyboxes & Openboxes are prone to losing channels on vertical transponders when another box connects to the same LNB.
There is a fix available on Ebay that connects between the affected box & the LNB cable.
Search Ebay for "no signal fix".
The spare LNB cable should be OK, as the seperate inputs on the LNB should be isolated from eachother. If you have any spare female to female back-to-back F connectors (used to join 2 cables together), then you could screw them onto the plugs to stop the inner connector from touching anything.