It's possible but not recommended as you would only have a limited choice of channels on one box whilst the other is being use if they share the same cable. In fact, it can sometimes not work and cause more trouble than it's worth.
This is because if one box is tuned to a specific channel and you try to use the other box that's connected to same cable (but split off) then this other box can only receive channels that are on same polorisation (H or V).
It would be best to run another cable from LNB, if possible.