Is there any way to have critical damage rolls roll as a single set of dice and output a cumulative total, instead of it rolling two distinct rolls and putting both, separately, to the VTT?

For example, I'd like a crossbow sneak attack (Rogue, lvl 4) crit to just roll 2d8 + 4d6, instead of 1d8 + 2d6, then a separate 1d8 + 2d6 again.

Beyond20 is unaffiliated with DDB. However, if you right click the damage roll, or long hold it on a mobile device, you can select to roll it as a crit, that way you can roll the weapon damage as a Crit once, and roll the Sneak Attack damage as a Crit once and yer done.

Quote from BurningBeechbone >>

Is there any way to have critical damage rolls roll as a single set of dice and output a cumulative total, instead of it rolling two distinct rolls and putting both, separately, to the VTT?

For example, I'd like a crossbow sneak attack (Rogue, lvl 4) crit to just roll 2d8 + 4d6, instead of 1d8 + 2d6, then a separate 1d8 + 2d6 again.

Your best bet is to make a "second weapon" - that is a Crossbow and call it Crossbow (Sneak Attack) and customize the area for additional damage to do 1d8 (Piercing) and then 2d6 (on top of that).

It's what I do for Sniper Shot for the Ranger in my group.

So something like this (quick and dirty didn't do the math, just an example) -

I spend a lot of time helping people on the Beyond20 Discord server. Probably your best bet for Beyond20 questions, specifically.