TV channel lost

2 UHF signals were combined & distributed, but at just one TV, only one of the channels could be received....

Always annoying to answer these without being there. 30 secs on site and I could tell you for sure. However:

1. Have you combined an aerial feed with the satbox signal by means of a splitter behind the TV set, or in any other way?
2. Some TV sets won't tune ch69.
3. Some TV sets have a fault that means they won't tune the lowest few channels, or the highest few.
4. Some TV sets won't tune a DSB signal, even though they have previously done so. They skip over it.
5. It is possible that a standing wave pattern is cancelling out one channel on one telly. This is especially likely if you have used a resistive splitter.
6. Are there any unterminated ends or cable discontinuities?

You could experiment by altering the output channels on the boxes, removing the splitter, etc.