The puffy tail is often because they're startled. If he was approaching the other cat slowly it's likely he's just investigating. I wouldn't worry unless there's crouching, charging, hissing or growling along with it.

Ooh, exciting adventure! If the neighbors are up for it, you could exchange small items that have each cat's smell on it (like a towel they've been sleeping on). Then they can kind of check each other out by smell before you let them meet again. Maybe get them together on opposite sides of a door first?

Gorgeous beautiful cat 💖

A puffed-out tail is an attempt to look bigger; it's at least a response to being alarmed if not aggressive. Be very wary if either or both begin caterwauling or growling; that's definitely hostile. And if his ears go flat, intervene immediately. Hopefully, right now he's just wary.

Kurt and Gunther will make a new friend very soon.

It's not a male/female thing, just territorial...but they could attack or just hiss/groan at each other. I'd be careful; if they think there's no exit they'll sometimes smack ❤

Kurti could have a new friend to play with. 😺

How beautiful you are today 💗💗

That is completely normal for first meeting ❤❤ your guys were siblings, but if you had already had one and had to introduce you would have experienced hisses and big tails. Recommendation is you slowly introduce. I always recommend to my patients to put up a baby gate between them and monitor closely and when they stop reacting thru the slow introduction you can remove the barrier and monitor. usually within a short time they begin to accept each other ….❤❤. Like people, some accept the newbie faster and easier than others. Cats can be very territorial…. So slow and easy introductions the best.

Meet and Greet with Kurt and Gunther. Da wär ich gern mal Mäuschen. 😂😂😂 Mal sehn wie schnell die zwei Schlafmützen sind. Schönen Sonntag aus Unterfranken. Nächsten Donnerstag, Flug und Familientag in Hammelburg. Kommt doch mal eingeflogen. 😂😂 zum Meet and Greet.

Wait until the other cat parent is present before letting them meet

Are these cats neutered? If so, you are not likely to see all out aggression. Vaccinated? Another worry; does the other cat have any communicable diseases? Talk to the neighbor and make these inquiries. Provide information as well. Exchanging smells, as noted below, does the same thing for the cats. Could be other cats on your floor as well.