That's not strictly true. I've found taking into account STAB and high attacking stats, a combination is often weaker than the two attacks used separately.
For instance, a Tyranitar using Stone Edge on a Rank 3 Def Pokemon does 16 damage, two would do 32. A Stone Edge+Stone Edge Combo only does 28.5, which is close but still weaker, not to mention energy costly. And that's with a 2.25 multiplier to BAP, iirc combos of different moves don't get this nice boost.
SE: 10+3+3=16
SSE: 22.5+3+3=28.5
Worth noting: There is a larger gap when using weaker moves in combo, 2 Rock Slides=28, Double Rock Slide=24
But that's not what I'm here for.
I'd like to look at multi-hit moves, because I think the way they work currently has some issues.
The current calculation method (bap*number of hits + stab + stat differential etc) works fine for normal circumstances, but there are instances where it poses a problem.
The main one is substitutes. Say the attack has enough power to break the sub and do some damage to the mon behind it. But how do we determine how much is absorbed by the sub? Do we just say it takes exactly as much as it had and the rest goes through? Or what? I really don't know how to handle that.
I personally think multi-hit moves should be calculated on a per-hit basis, as in find the damage for one hit and multiply that by the number of hits. Obviously there would have to be some nerfing of the effects of stab and stat differential (I was thinking reduce STAB to 1, similar to crits) but I think overall it would be a logical change.
You could then work out how many hits breaks a sub, and how many hits go through to hit the mon.
It has other applications too, eg to hit multiple targets. "Cloyster use Rock Blast to hit all of the clones and the real Charizard!"