No, no it isn't. Not at all. Shared exp is never the most efficient way, it would be one of the least efficient methods to train.
The most efficient way to train would be like this
Train your strength up to a maximum hit, go up one level so that maximum hit carries over to non-aggressive attack styles (aka non-strength training modes) and then train attack up to the same level as your strength.
Repeat the process.
Once you get to the max hit for whatever weapon you're training with (usually 96-98 str (only some are 99 like the abyssal whip)), then you prioritize attack to 99 and then go back to defence to train from 1.
That is the fastest way to train to max melee.
If you're saying efficient as in how much you spend to get the stats, then you would do what I said above, however you would then level your defence to 70 after getting roughly 75-80 attack and strength. Then get some actual gear and then keep going with what I said above.
Defence is always left to last for the most efficient training methods.