Thinking of all possibilities, and you can see if this is the case by swapping inputs as JamesO suggested, one of the pigtails may have a funky connection. Sometimes when cables are made up, there isn't the best of connections.
I'd also suggest seeing what values you show by connecting just one scanner at a time. They say that running two scanners off one antenna may not be the best method - signal could be robbed from one to the other.
There are multicouplers available which are made to connect multiple scanners to one antenna. These devices have, I understand, some type of RF amplification circuit.
We used a multicoupler at our fire station as we had several scanners running from one antenna mounted on a tall tower. It was fed with real good cable.
At home, I have a couple of scanners in my garage. Each has its own antenna in the attic over the garage. I figured that it was cheaper to use two antennas than to buy a multicoupler. If I had more scanners, then I'd run the finances to see whether a multicoupler would be a less costly endeavor.