I don't understand the need for a combined roll. The save roll isn't complicated or taxing, it adds an element of agency for the other player and it adds a dynamic of the armour itself, which is extremely important in 40k, with everything between terminators and t-shirt saves. I can't see why anyone would want to combine the roll and get rid of armour saves. Also in a D6 game its impossible to express the type of armour in the game with a D6.
It's not hard at all actually, you just need to play with statistics. The first and foremost largest problem 40k faces currently is dice roll bloat; the game has too many god damn dice for too many weapons. Ideally, both for our sanity and shortening game time from the insane bloat, one model should only ever get one roll, and possibly that should be reduced as well to something more akin to apocalypse (or lion rampant) - a unit has a degrading profile based on the losses it sustains. One thing I would absolutely scrap as completely nonsensical is weapon differentiation. There is no point in classifying missile launchers, lascannons, multi meltas, fusion guns, etc as different weapons.
All we need is three classifications of weapon type - light, medium, and heavy. Tactically and strategically, a lascannon or meltagun do not differ enough to justify different statlines compared to how they would perform in combat. Same goes with melee weapons. We don't need thunder hammers, power fists, scythes, swords, chainswords, etc. We just need light, medium, and heavy melee weapons. How you model your units really shouldn't matter as what does matter is how the weapon is used by a squad, and not so much how that weapon works. It'd be like having different rules for different small arms in historical wargames - the stopping power of an MP44 vs an M1 Garand don't matter on the scale we're trying to represent. What matters is if the squad is suppressed, taking casualties, or if what's being fired at them can even hurt them.
I wouldn't mind fewer rolls if it also means dice with more sides. Like a d12 or such. If we stick with d6 I really don't want things to be more swingy.