Thank you for answering… That already better than 90% other posts I ever posted on this forum 
But I’m looking for examples of how to do it with Soapy. Not the information it can be done. It is clear that it can be done, but I never found any examples let alone explanation how it works. Sure one can spend a week of work going throught the source. If this was my job or I had more spare time I wprobably would, but my hope (probably futile seeing how this guy: Dual RX with SoapySDR on different frequencies - #6 by KarlL never got an answer in March 2018) is that 5 years later this knowledge is slightly more widely disseminated and someone will feel like sharing.
I have no difficulty with WDSP. That part has already been written and works great with different hardware. I just need to give it/receive samples. It is up to me to decide how. Soapy or any other way. Soapy seems the most mature (and a lot of the code is already written and works for other SDRs that support soapy). So all I’m trying to do it to make it work with Lime.
What this post is specifically about is how Soapy sets frequencies. How does it decice LO frequency when two RX (or an RX and a TX channel) request different frequencies? How do I force a single LO ? Should I use the OFFSETT and the RF method or the BB method? BTW I’m using C not C++, but if someone has C++ code that also would be very useful.
TX and RX sides have their own synthesisers and are supposed to only share the reference frequency so why is it that the TX LO jumps around when the RX frequency is changed with soapy?
Unfortunately (unless I missed something) the file you mention - fftReceive.cpp doesn’t seem to use the API I ask about. It setts an offset with NCO by using some API unkown to me (“liquid”?). I have no interest in adding another dependency. I’d like to figure out how SOAPY does what it does so I can use it effectively.