Even having a slight increase of spawn monsters and them having 1.25 more health would be effective maybe since it will mean more things you need to shoot at together? I dunno, just doubling it seems to be more effort if you both aint shooting at the same thing.
You're assuming two players -- and that spawning more monsters would actually work (it wouldn't, often). Generally speaking if you had, say, 8 players and 8x as many monsters then you would find it unplayable. Or pick your numbers, it just simply doesn't scale.
Regarding monsters taking longer to kill in MP, that's simply not true. Simple math:
- If player A does 1 unit of damage, and monsters X and Y have 2 units of health each, then in solo it takes player A 4 shots to kill them all. So, 4 shots per player, since there's one player.
- If players A and B both do 1 unit of damage, and monsters X and Y have 4 units of health each (thanks to MP buff), then takes the players 8 shots to kill them all. So, once again, 4 shots per player.
If players split up and fight different monsters in the same chunk, then yeah it takes twice as long to kill each one with two players; but the number of shots fired to clear the chunk remains the same. It's possible we might do some sub-linear scaling of monster health in the future, but that's more a matter of simply having to cave to very wrong perceptions that people have rather than any basis in real balance.
The simple fact is that having a friend helps enormously already, even with perfectly linear scaling (common misconception of "why have the friend there at all"). If you and your friend stand right on top of each other, then both of you can be hit by enemy shots, sure. But otherwise the enemy can only shoot at one of you (or the chasing enemies can only chase one of you, etc). You can flank enemies and shoot them from multiple directions, and ultimately overwhelm them if you play your cards right.
So you wind up getting into extreme "takes much less damage" situations even if you're good about it.
Perception matters, obviously. But in this case people's perceptions about the math of it are woefully incorrect and that kind of saddens me; it's so well balanced right now, but people are rejecting it because of doing bad math.