Swamp Thing and The Toxic Avenger is the only thing that comes to mind at the moment. I'll think of something better later.

When in doubt, look to Italy.

The Warriors / 1990 Bronx Warriors
Escape from New York / 2019: After the Fall of New York
Dawn of the Dead / Zombi 2
Indiana Jones and the Temple of Doom / Massacre in Dinosaur Valley

also

The Evil Dead / Evil Dead Inbred Rednecks
Rambo III / Deadly Prey
Curse of Frankenstein / Flesh for Frankenstein
The Texas Chainsaw Massacre Part 4 / The Butcher Boys

Mainstream: Christine (1983)
B movie: The Car (1977)

some of these dont work but hey, its fun

Resovoir Dogs/Hip Hop Locos
Irreversible/Savage Streets
Apocalypse Now/Combat Shock
Psycho/Re Animator
Dead RingersA Zed & Two Noughts

Hostel/Bloodsucking Freaks
Deliverance/Rituals
The Shining/Motel Hell
Interview With a Vampire/Mr Vampire
Arrival/Evil Aliens

City of God/The Warriros
Guradians of the Galaxy/Starship Troopers
Rope/Murder Party
Alien/Alien Apocalypse
..erm
PeeWees Big Adventure/Antikorper

I fail to see the connection between Psycho and Re-Animator?

I would say
Psycho / Homicidal
Re-Animator / Creature with the Atom Brain (1955)

yeah that one is tenuous - it's the theme music/score. basically re animator is bernie hermann on speed, suprised there was never legal action. Also, im pretty sure West has some momma issues!

do like your alternatives, nice job

The Cabin in the Woods
Resolution

The b-movie equivalent in this case is much better than the mainstream movie. I think Justin Benson and Aaron Moorhead are dope, and I want to see The Endless.

I agree completely...Resolution is the film TCITW wanted to be. I think Resolution might be my favorite film of the 20-teens.

I've seen these two films compared before, but I don't know why. Other than the setting, I don't see the connection at all. They're nothing alike.

Mainstream: Jaws
B-movie: Piranha

or does it have to be 2 movies I like, I'm not a fan of Jaws at all. I think when it comes down to creatures B-movies are more fun than mainstream.

Ok this time when I like both

Mainstream: The Innocents (1961)
B-movie: Tormented (1960)

Mainstream: Kingdom of the Spiders
B-movie: Earth vs the Spider or Tarantula

I guess Cannibal Holocaust is a B-movie can't imagine this being shown to a mainstream audience. Are there actually decent mainstream cannibal movies, can't think of one. Green Inferno I found pretty rubbish.

Alien and any number of crappy Eye-talian rip-offs. Could even say The Deadly Spawn or Without Warning, two American movies, are the B-movie equivalents, despite the fact that their storylines and settings are much simpler and um....errr...regional.

Kinda hard to think of many examples since many mainstream movies are b-movies themselves, at least in the glory days of 20th century horror movie making.

Friday the 13th is very mainstream, but is it a b-movie? The first one, at least? Maybe a Friday the 13th/Sleepaway Camp double-bill.

F13 definitely should be a b-movie for all intents and purposes, but since it was Paramount, a big company, they pushed the hell out of it until it became mainstream. It definitely blurs the line on what is, and ought to be a b-movie. Sleepaway Camp is definitely the more obscure of the two, so in this case, it's perfectly valid. It's all relative, I guess.