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I want to exclude superheroes who were in one team and later in another (I'm thinking in Hulk, who was an Avenger first and a Defender later)
Only the first hero who was active part of two teams at the same time.
I will accept answers that are based on Marvels fictional timeline, but I prefer answers with the comic book's publication date.
There is a related question here:
Beast is a founding member of the X-Men in X-Men #1 (Sep 1963). He remains a member of the team until they stop printing new issues as of issue #66 in March 1970: https://marvel.fandom.com/wiki/X-Men_Vol_1_66
It's mentioned in passing in Giant-Size X-Men #1 (May 1975) that Beast had at some point left the X-Men. https://marvel.fandom.com/wiki/Giant-Size_X-Men_Vol_1_1
Beast joins the Avengers in Avengers #137 (July 1975) https://marvel.fandom.com/wiki/Avengers_Vol_1_137
He leaves the Avengers in Avengers #211 (Sept 1981) https://marvel.fandom.com/wiki/Avengers_Vol_1_211
Beast joins the Defenders in Defenders #104 (February 1982) https://marvel.fandom.com/wiki/Defenders_Vol_1_104
He stays with that team through the remainder of its run, before they disband in Defenders #152 (Feb 1986) (a Secret Wars II tie-in). https://marvel.fandom.com/wiki/Defenders_Vol_1_152
From there Beast immediately joins the re-formed X-Factor in X-Factor #1 (Feb 1986) along with his long-time Defenders (and previously X-Men) teammates Angel and Iceman. https://marvel.fandom.com/wiki/X-Factor_Vol_1_1
Beast leaves X-Factor (the team reforms entirely, kicking off one of the best runs on any Marvel book of all time as Peter David takes the helm with a brand-new, entirely dysfunctional group) in in X-Factor #70 (Sep 1991) and re-joins the X-Men in X-Men #1 (Oct 1991) https://marvel.fandom.com/wiki/X-Factor_Vol_1_70 https://marvel.fandom.com/wiki/X-Men_Vol_2_1
It turns out that all Hank's team memberships throughout the 70's and 80's were entirely sequential and never overlapped. I'm actually quite surprised.
In Avengers #300 (Feb 1989), during the Inferno crossover event, Sue and Reed Richards apparently joined the Avengers for a while: https://marvel.fandom.com/wiki/Avengers_Vol_1_300
However, they were not in the Fantastic Four at that time, cf. https://marvel.fandom.com/wiki/Fantastic_Four_Vol_1_323
So this is a near miss - even though the cover art does show them in the iconic FF unis.
Ben was an official member of West Coast Avengers in West Coast Avengers (vol 2) #9 (June 1986). https://marvel.fandom.com/wiki/West_Coast_Avengers_Vol_2_9
But this occurred during a period of his absence from the FF (replaced by She-Hulk): cf. https://marvel.fandom.com/wiki/Fantastic_Four_Vol_1_291
Marvel relaunched the Defenders as Secret Defenders in March 1993 where Dr. Strange recruits various teams of heroes to face individual threats. Wolverine is featured in the first three issues. Assuming one counts this as superhero team membership, this is another Wolvie cross-team situation. https://marvel.fandom.com/wiki/Secret_Defenders_Vol_1_1
...And then I remembered this one. One of the few times my young self bothered to purchase a few issues of FF.
A Skrull KO's the entire FF and recruits a "new Fantastic Four" to serve in their stead, leading to a three-issue arc where Hulk, Spider-Man, Ghost Rider and Wolverine comprise the team. https://marvel.fandom.com/wiki/Fantastic_Four_Vol_1_347
Wolverine is still in the X-Men as of this period (cf. Uncanny #271 https://marvel.fandom.com/wiki/Uncanny_X-Men_Vol_1_271).
So it may or may not be the first instance, but at worst we have an endpoint from which to work backwards. On another personal note, this arc features some really incredible pencils from Art Adams.
I couldn't find any instances earlier than this!
If you you consider the Inhumans a "Superhero Team" (I do not), then the answer is Crystal, who officially joins the Fantastic Four in FF #81 (Dec 1968), presumably coincident with her 'membership' in the Inhumans. https://marvel.fandom.com/wiki/Fantastic_Four_Vol_1_81
Two possibilities:
1) It's possible that The Beast (Hank McCoy) didn't stop being considered an X-Man when he was in the Avengers;
2) Mr. Fantastic's brief stint with (again) the Avengers, but that was much later.
