The LNB has 4 states, 4 "bands" if you like which it has to flick between, and is controlled by the tuner in the box. You CAN split a single satellite feed but only channels on the band which the LNB is tuned to will work. The easy workaround is to have 2 separate LNB's controlled independantly (usually built into a single "dual" LNB, which is just 2 LNB's built in the same package), so the tuners can access any frequencies across all 4 bands as they wish.

Also remember that simply splitting the cable would overpower the LNB as it is powered by a current sent up the coax from the receiver, so a special splitter is needed which doesnt duplicate the power feed and will designate one tuner the master, thus only sending commands to the LNB from that one. The slave tuner only gets access to the band the main one happens to be on.