With what appears to be no isolation based on the post, I figure the OP is using a simple T connector. Thus, the interference and poor receive signal. Perhaps connect the discone directly to a single scanner and see how much it improves.
@Rawkee1 - By the way, what did you use for an antenna before and was it also above your roofline? How are your two scanners getting power?
Additionally...
1. The OP, like many others, probably should use an FM broadcast filter inline on the coaxial cable. FM broadcast stations use a high amount of wattage that can significantly interfere with receiving signals. Yes, FM broadcast stations on 88~108 MHz can interfere with VHF and UHF frequencies. An FM filter costs anywhere from under $20 to around $100. See the link to the thread in my signature below.
2. Use LMR-400 and it will reduce your coaxial cable losses. I would put an FM filter inline first as that may fix things to your satisfaction.
3. Additional RFI (Radio Frequency Interference) can be caused by strong signals from paging systems around 152 MHz. NOAA WX signals around 162 MHz, or by AC wall warts in the home, or various electronic devices. If that is also an issue, filters can be purchased to help resolve/reduce that RFI.
Radio Frequency Interference (RFI)
4. The eight foot height above your roofline should be fine, as long as there are not any nearby structures that are above that antenna. My neighbor has a two story house, so my antennas are above her roofline.
