You really need to get this:
25-HFS21 JVI Satellite
Put you new receiver on the power-pass leg and the old one on the DC blocked leg of the splitter.
That splitter you put in is allowing DC to pass from both receivers to the LNB (and to each other!) and causing the problems you are describing.
In any case, you shouldn't continue using this setup. With two receivers hooked together like this, you could conceivably damage one or the other of the systems. When one receiver is trying to tune one polarity and sources 18V to the antenna, the other one may be tuned to another polarity and instead tries to source 14V to the antenna, you are going to get a net current flow out of the 18V source to the 14V source.
The receiver at 14V will have to sink some amount of this back current, possibly damaging components. The receiver at 18V will probably be OK, but it also could be damaged as it is trying to send current into a lower- voltage source.
By putting the power-pass leg of the splitter to the new receiver, it will function 100% all the time. The old receiver will only function correctly on the channels that happen to match the polarity that the new receiver is tuned to. Basically, you'll tune about 1/2 of your channels at any time, and if you want to tune the other 1/2, you'll have to change the channel (polarity) of the new receiver.