EzekielRaiden
Follower of the Way
Oooor...you can just do the really simple thing of "maximize base damage".You get better results if you add up the dice, the bonuses, and everything else, get the original damage total, and then multiply that.
So if your 2x-crit roll on a sword attack is d8+6 and you roll 2, you end up doing 16 damage.
It's not even hard to calculate. 2d6? 12. 4d8? 4x8=24. It's extremely easy to maximize the (rolled) damage.
And, as noted, the average result of "roll XdY twice and add" is X(Y+1). Naturally, you'll miss out on the handful of times you could've gotten an utterly ridiculous crit, e.g. 2XY, but in return you avoid the anemic ones.
Also, doing the above actually can result in some significant problems, because it means static damage bonuses are extremely powerful. Consider someone using a greatsword (2d6) with Great Weapon Master and a +5 Strength bonus. A crit then becomes (2d6+11)·2, or 2·(2d6)+22, which gives a spread of 26-46 inclusive, average 36. At least half the damage from that attack--in almost all cases--comes from the static damage, not the rolled damage. And this is using the 5.5e version, it would be worse with the 5.0 version, a whopping 2·(2d6)+30, so at least 60% of the damage, even if you roll the max value, comes from the static bonus alone.
Now imagine if we'd tacked on stuff like a +3 magic weapon.
Maximizing the rolled dice for a critical hit really is a sweet spot of simplicity, impact, and reasonable balance. Heck, one could even argue that it actually hearkens back to an idea from yesteryear--attack matrices, which were just precalculated to-hit tables--and to an idea that 5e actually does use anyway, which is letting players take the rounded-up average result for determining HP by level (which, in my experience, everyone does in fact take that option; I'm sure some people don't, but I've never played with anyone who does.)
It's extraordinarily rare when something manages to be simultaneously simple, effective, and balanced. We really shouldn't ignore it when such opportunities present themselves.