Going to post the maths here. Going back over it my OP is wrong but it's still... really not much different.
MELEE 76.8 ⇒ 75.4 (-1.4)
CASTER 38.4 ⇒ 37.12 (-1.3)
CANNON 120 ⇒ 116 (-4)
In a duo lane you get half of that XP.
MELEE 38.4 ⇒ 37.7 (-0.7)
CASTER 19.2 ⇒ 18.56 (-0.64)
CANNON 60 ⇒ 57 (-2)
Every minute you have 6 melees, 6 casters and 1 cannon minion*.
This means that you're losing 10.04 XP per minute.
One cannon wave unshared is worth 363 XP.
...or 36 minutes of XP.
So there you have it. It's not 72 minutes like in the OP (rather 72 waves). It was some 3am for me when I posted.
*this changes over the game but XP pacing is largely irrelevant by the point you're having more cannons spawn and early game no cannon minions spawn which makes the changes less impactful.