i am in africa have dual lnb which provides me free view from middle east etc al jazeera etc i feed to coaxial cables to different decoders but only one works well some channels say no singal on one decdore and one works fine when i move on the other dedcoder if i move the channels the channels that were showing no singal show up what do i do they don t both work at once?

The receiver will, on a carrier by carrier basis, be able to use +13 or +19 volts DC and 22kHz tone on or off.  This will affect the LNB polarisation and local oscillator frequency.

Without technical spec, I don't know what is your 'dual LNB'.  Try to find out how your 'dual LNB' works.

If your 'dual LNB' has two feed horns then the distance apart must correspond to the spacing between the two satellites and the focal length of your dish.

Best regards, Eric.

its one lnb with two connectors to feed in two cables the other dual viewer can only view the channels when the 1st user is viewing the same channel only vertical channels  do this when the 1st user changes to a horizontal channnel the vertical channels on the 2 users dies out

Disconnect both cables at the LNB end and use a DC voltmeter to check the voltages coming from the receivers.  Each receiver should produce +13 volts and +19 volts according to what polarisation is wanted.  A faulty sheath connection on one cable might produce the symptom you describe.

If the volts are normal and change then either the LNB is faulty or it was not designed to operate on both polarisations simultaneously.

You don't mention frequency bands.  Satellite TV Ku band LNBs often have low/ hi band local oscillator selection (9.75 GHz / 10.6 GHz) using 22kHz tone.  This is a further complication that may or may not be designed into the LNB.

Best regards, Eric.