The dish is fitted with an LNB which is, probably, a single LNB which can be either switched to detect Horizontal or Vertical polarised channels. Meaning that only half the channels will be available at one time, whichever way the controlling box has switched the LNB. The LNB, but not the dish, will need to be replaced with a twin LNB, also needed to feed a Sky+ receiver with 2 internal tuners. This has two outputs, with a cable running to each satellite receiver. You would be better off with a quad LNB which, in the longer term will support 2 Sky+ boxes.
See
here
Also means running additional cables from the dish/LNB to each receiver.