Maplins supply a splitter, obviously it has limitations, but they do exist.
"feed two receivers from a single LNB. One of the outputs has DC pass, and the receiver connected to this supplies power to the LNB. In the case of most systems, this ‘master’ receiver will also select polarity, and thus determine the channels available to the slave receiver. Although an inexpensive method of running two receivers from a single dish, it is not as flexible as a twin LNB which allows two receivers to select channels of either polarity independently. Frequency range: 5MHz to 2GHz. To protect one receiver (and its user) if a fault should develop in the other, this splitter has screw terminals for earthing."
The only way to run two boxes totally independent of each other is to have a twin LNB which is effectively two LNB's in one case. You may find a Quad LNB cheaper and give you two backup feeds as they are the standard LNB used by Sky.
Sorry but you STILL need multiple LNB's. These products only solve the use of multiple cables, which in certain circumstances can be a problem. However you still need a multiple LNB on the dish and then use a Stacker to put two signals down the same cable. I don't think that is the problem here, the OP wants to use a single LNB to feed multiple boxes.
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