I literally just started trying to play tenor and bari at the same time (like over the last hour).  My first task was customizing my harness, but I still need more stability.   I can get air through both but need more bari mouthpiece in my mouth to get equal sound from both.  Any embouchure tips?  I've arranged a part for this particular case so that's no issue.  Executing it is.  (New here, don't crucify me yet, lol!!). Thanks all!!!  KA

My advice is learn to play one saxophone really well, then play one saxophone. 

Roland Kirk used to play three saxes at once.  He apparently had an enormous mouth in order to hold three mouthpieces.  I tried this years ago with a tenor and an alto.  Using the uuper stack I was able to simultaneously play Sentimental Journey.  However, except as a stunt, I can't imagine why you'd want to do this - it doesn't sound very good as I recall.

If you're determined to do this, your left hand will play the upper register and the palm keys.  This will let you run the scale from G to F in the upper register and G to C# in the middle register.  You'll need the octave key for the upper register, of course.

Now the tricky part is playing the upper stack on the second horn with your right hand.  You'll be able to play the lower and middle register from G up to C# and the high F.  By pressing the high F key without pressing the C key and using the octave key you can get a high E.  The palm keys will be out of reach.  You can press the octave key with your right thumb.  BUT, both horns will swing loose from the neckstrap and you'll loose some control over your embouchure as a result.  Having two mouthpieces inside your mouth at once is likely to make the resultant sound less than pleasant.  Good luck with your stunt!