Splitting the feeder cable from any type of aerial or dish inevitably involves a certain amount of signal loss. When splitting the signal from a terrestrial antenna that loss is usually not so great as to make the resultant two signals unusable. However that's not the case with satellite reception. You'll still get a very weak signal along each cable but it won't be strong enough for the digiboxes to use.
However you can still use a single dish to feed two boxes. You simply need to replace the LNB (= 'the sticking out bit') with a dual LNB, together with an extra cable.
Terrestrial TV transmitters are high-powered (typically around 250,000 Watts) and usually a few miles away from the receiving aerial. Satellite transmitters are very low-powered (typically around 30W) and at least 22,000 miles from your dish. So it's unsurprising that you can't get a strong enough signal, via a single LNB, to feed two boxes.
Chris