According to the link you posted to NWR coverage in Colorado, Douglas County is covered by two transmitters using the same SAME code:
Code:
Douglas 008035 Denver 162.550 KEC76 330
Douglas 008035 Franktown 162.450 WNG550 300
And Wells County is covered by four transmitters:
Code:
Weld 008123 Fort Collins/Ault 162.450 WXM92 300
Weld 008123 Greeley 162.400 WXM50 300
Weld 008123 Mead/Longmont 162.475 WXM51 100 Southwest 1/4
Weld 008123 Sterling 162.400 WXM53 300 Northeast 1/4
To get the alerts for those counties, program the transmitter you can hear best. If you can't hear a transmitter that is programmed to broadcast the SAME alerts for the county in question, you won't get the alerts. Also, note that while 162.450 covers both counties, they are not the same transmitter.
By the way, I'm not aware of a weather radio that will scan multiple transmit channels while also listening for SAME codes to decode. If you need to monitor multiple transmitters to get the SAME alerts of interest, I believe you'll need mulitple weather radios.
As for the specific phenomena you experienced, you may want to call the nearest NWS office and ask to talk to the Warning Coordination Meteorologist. He can probably answer your question as to how the transmitters are programmed.