trying to play two radio stations simultaneously

Hello All,

I'm trying out jean-michel.friedt's course on digital communications
and have a problem with getting two radio stations to play
simultaneously. I have tried to follow jean-michel's example, but am
getting caught somewhere. I get one radio station and hiss on the
other.

I've tried a few combinations, but haven't got a logical successful
solution. I am choosing two local radio stations and can tune them
independently. The first radio station is the osmocom source ch0
frequency. This station I can tune without trouble. The second
station is determined by s frequency translating filter and I make the
center frequency a negative difference between the ch0 frequency and
it's nearest neighbour. In this case I'm choosing ch0 frequency
90.7e6 and center frequency -800e3. (For a radio station at 91.5
MHz.) Somewhere here I am getting into trouble. This station can be
heard but is mostly hiss.

I'm including my grc file and a screenshot of my flow graph with the
hope that someone can point me to my error. Thanks in advance.

Chris