Whilst you can get splitters for such applications, they are specialist items and not for use in general circumstances.
This is because satellite receivers output voltage up the cable to the dish, to power the LNB (the bit at the end of the dish. And what is more, they output two different voltages, to select the vertical or horizontal antenna inside the LNB, and a 22KHz signal to select the high or low band of frequencies.
So if you simply put a splitter in, the LNB has to be in one of four modes for each receiver - which is, of course impossible. And each receiver also has to be protected from the output of the other receiver.
So - the answer you need is to fit a double LNB onto the dish - this has two independatd connections (as used for Sky+ boxes), and you then run a cable to each receiver.