i found a very convenient yet productive way to perform super sets,this way involves working the same muscle.some seloctorized machines have more than one grip[handle] usually a straight handle and a parallal handle,lets take for example the shoulder press machine,the seated row machine and the vertical chest press machine,on any of those machines i would start with the straight handle and do as many reps i can[go to failure]then imedietly and without changing the weight,i would go to the parallal handels and get as much reps as i can,the reason that i can get few more reps with the same weight that i reached failure with on the previouce set,is,that with the parallal grip the secondery muscle gets involved more in the movement,for example with the back row,the biceps kick in and with the shoulder and chest movements the triceps kick in.give this a try.