Secondary 02 may not be true WB02. They just tell whether the cat is working or not. Assuming you mean you have removed one of them and reused the exhaust bung for a real WB02 sensor for your gauge, and have a tune that is set up for no cats, you may still have a problem. I think the tunes simply alter the signal, but expect the heater circuit to work and the signal alteration is based on it being in the exhaust pipe. If so, you may have to add a third bung if this is the way you want to do it. Vibrant sells stainless bungs for about $5 and you can drill the pipe and mark it for another bung and take it to a welder.
The easiest thing to do would just be to read your primary 02, which are real WB02. There are a few ways to do this. Right through the OBD2 is one way. A piggyback is another way. I can read my factory WB02 right through my Vishnu tune, having it displayed on the cluster.