I dont allow it in leagues I admin because I simply dont see how you fully concentrate and dedicate yourself to multiple teams competing to win.
In free agency if both teams could use the same player...which team do you offer the better contract with?
If they play each other...then what?
On the waiver wire which team do you use to put in a claim?
If one team is on the verge of the playoffs and the other is out of the playoffs, does the team out of the playoffs play games against opponents that would help his other team with high fatigue settings to ensure his best players remain on the field with no regard for injuries?
If each team were in different conferences, would they not offer trades to teams that impact each others teams? For instance, an NFC EAST team wouldnt offer to trade a star player for draft picks to a team in the AFC WEST if his 2nd team were in that divison even though it's the best option for the NFC team.
Trades can be monitored and voided and simply banned for that owner between his 2 teams but my biggest concern is for owners that have multiple teams with zero transparency about the fact.
I'm in a league where the same person owns 2 teams and even though everyone was made aware of the fact, he continues to own both teams. The bizarre part is when he posts something as the owner of one team and either likes it or comments as the owner of the 2nd team. It's my understanding that owner use to be in this league and someone like that you dont want with one team let alone two in your league.
It's my opinion, you cant fully compete to win if you own multiple teams in the same league. You would eventually favor one over the other. There are plenty of teams available in a wide variety of leagues to own where nobody should have to own multiple teams in the same league and have any of these conflicts.
Just my own personal opinions