All valves open full and back quarter turn. If you have a failure and cannot determine which side it is coming from, start with the right post. Shut it down and breathe the primary regulator down and switch to backup on left post. This way if the sound of bubbles stops, you know it has to do with the right post regulator. If it continues with the right post off, then shut the isolator. It will take a bit for the tank to empty, so you will hear bubbles for quite a while.
In the mean time you should have gotten your buddies attention by now and he can inspect to see if it is a manifold failure or if it is actually a left reg failure. At this point in time he will ensure that you are on the correct regulator (making you change if neccesary) , ensure that the failure is isolated, inform you of where the failure is and turn or thumb the dive. At this point you will do a quick check of your valves to ensure they are in the proper position to isolate the malfunction (based on what your buddy communicated to you), check pressure (if it isn't a left post failure) and head back or up.
If you can hear it is a left post failure then start with the left post. Basically for the sake of making this second nature, if the failure point is unknown start from the right and work your way left.
If it winds up being a right or left post failure, you are not going to benefit from starting with the isolator. Plus the amount of gas lost will determine if the dive can be continued or should be turned or thumbed.
And it all comes down to practice, familiarization and standardization with your equipment (you and your buddy) and maintenance of the gear.
