Can I split one coax lead from the dish or multi-switch to two receivers or to a receiver that has two inputs?

The only time that you could split a single LNB line is to use a special High Frequency splitter that passes power to one side, the one side that passes power would be the master receiver that determines which satellite both tuners are to receive signals from. In a hotel or sports bar this may work if all channels needed are coming from the same satellite, however for home use where you want to be able to watch every channel that your paying for on both receivers or two tuners in one box, this method would not work, if it were possible to do this there would be no need for dual LNB's you would just split the signal.

 A receiver sends 13 or 18 volts to the LNB to pick one of two satellites, that's why you need a dual LNB if you have two receivers, each receiver can send 13 or 18 volts to the LNB, with a dual LNB you can be watching channels coming from two different satellites.

You need to use a multi-switch to get the extra lead, a multi-switch is just that, a switch and not a splitter, the receivers will power the switch and the switch will send 13 volts to one side of the LNB and send 18 volts to the other side of the LNB, the LNB will send signals from both satellites to the switch, now each receiver can send 13 or 18 volt to the switch and the switch will connect each receiver to the correct side of the dual LNB depending on which channel the receiver is trying to pick up. (In the case of a receiver with two tuners the receiver needs two inputs from the multi-switch)