Straight from the DMG Errata:
Attunement (p. 136)
The first paragraph ends with a new sentence:
“If the prerequisite is to be a spellcaster, a creature qualifies
if it can cast at least one spell using its traits or features, not
using a magic item or the like.”
emphasis added
Therefore, for the purpose of attunement:
If a creature is able to cast a spell by virtue of their own ability (not from an item), whether that be race, class, or feature, you are a spellcaster.
It should be fine, as long as time allows.
Nothing in the rules says that a long rest cannot immediately follow a short rest. In this case, what the characters are doing during those rests are very different.
Indeed, a short rest and a long rest are what many real-life people do before bed: an hour or so of non-strenuous activity, like reading, and then going to sleep. It's not a stretch to say that a D&D character can't spend an hour studying their magical item, and then go to sleep.
The only time where that might be a problem is if the characters don't have the full 9 hours.
Different things are happening during the two rests.
What's really key here is that the activities during the two rests are different. In the short rest portion, the character is studying the magic item, meditating on it, or whatever is required for attunement. In the long rest portion, the character is doing something else, such as sleeping. The attunement process is still active work, just not active relative to adventuring.
I think that this distinction is why attunement is limited to short rests, and why a character should be able to do chain them together.
Best Answer
"Last attuner wins"
Yes, it really does work like that (emphasis added):
So yes, if a character attunes to an item, it breaks any existing attunement to that item by another character. In other words, yes, you can "steal" attunement from another character.