How EXACTLY does the two-player mode work in Cuphead?

Disclaimer: I've been searching over and over for a detailed explanation of how does the two-player mode work in Cuphead, but there are tons of results linking to completely worthless articles from garbage sites.

So far, I've found exactly zero useful information, so I'm asking here.

So, is there an official or informed source regarding the 2-player mechanics?

Specifically:

• when there are two players, do the monsters gain more lives, more attacks, or what?
• if someone dies and fails to be revived, what exactly determines when he can come back? I've noticed that sometimes it's almost immediate, sometimes it's very delayed. Sometimes pressing start makes you come back, often it has no effect. No clue.
• is there any non-obvious effect? (i.e. not like "both have coins to spend", ok thank you very much, I can see that…)

While technically these look like multiple questions, THEY ARE NOT. The question is: is there a list of the actual mechanic differences in the 2-player mode? Those above are just examples of what such a list should include.

Solution 1:

The only major differences are the ones you already list:

1. Boss HP is doubled when in 2 player mode, monsters are unaffected.
2. If another player dies and is not revived by performing a parry on their pink heart, they are out of the round and will not return until wipe or stage completion. The can steal a hit from the partner player and revive if they have a heart to spare to the dead friend.

Other than that, it's Cuphead as normal. Both players can pick weapons/abilities separately and everything else is the same as it would be when playing single player (enemy/boss patterns, stage setup, etc.)