So if your'e using Bromine, your chlorine levels will be non existent and visa versa
Though technically true, test kits will not show this because they do not distinguish between chlorine and bromine. Test kit indicator dyes react to both. The only difference is the units of measurement where bromine is 2.25x the chlorine level though test kits normally just show 2x if shown on the same comparator tube.
Also, one can use chlorine to activate bromide to bromine in a bromine spa so in that sense they can be mixed. Also, the most common bromine tabs, BCDMH, are bromine and chlorine combined in the same tablet, but the chlorine will activate bromide in the bromide bank to bromine.