try to measure the vacuum pressure first

Usually, if the catalytical converter is bad, the vaccum will be decreasing when idealing, as a result of back pressure due to CC blockage. If the Vacuum is normal and no decreasing trend, then measure the O2 sensor difference before and after the second CC. If they are close to each other, I would suggest the swap them and measure again, as this way you will know if the O2 sensors are good or bad (assume the possibility of both going bad is very small). If everything works fine, it has to be CC problem.