I've always said the the true greatness of a fighter is the ability to reign over a division and win fights at the highest level, which are title fights. The more a fighter does this, the greater they are. It is the most honest way to prove greatness. If we look at the the long reigning champs like Jones, Anderson, GSP, Johnson and then the next level guys like Usman, Volk and Izzy, these guys have shown the ability to beat most of the best fighters in a period of time in a division. All those guys were the best for several years.
Now in terms of two division champs and how that stacks up against single division champs? In theory, if you have a long time divisional champion who defends 6-8 times and then moves up and defends several times in a different class. Yes, that would be IMO be more superior than a single divisional champion. And the UFC has attempted this with Izzy and Volk, but they just couldn't win the titles.
However, majority of the two division champs and ones that are trying now falls more into just the right match making and path to titles and I see it more as fabrication greatness and marekting than actual greatness based on wins. Guys like Conor, Cejudo, Perreira fall right into that. They got the right path to title fights and it's more fabricated than legitimattly being great.
** please excuse my little side rant, I address your exact subject matter below.
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It is funny to me how Dana used to go on and on about how his business model was superior to boxing, only for his company to then go PRECISELY down so many of the same paths.
The UFC has 2 heavyweight champs right now and they aren't planning to fight eachother next.
"Our guys fight 3x a year against the best guys"
Outside of PrideFC, they never did, but currently we are lucky to see a belt defended every 10 months or so, and rematches happen so often that a division can easily go 2-3 years without fresh challengers, as we have seen in HW, 125, LW (Islam hasn't faced an actual LW in 1.5 years I think)
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The whole "Double Champ" nonsense is almost entirely based on favoritism.
Jose Aldo didn't speak English so when he had 6 title defenses he was told he needed to vacate to attempt 155, meanwhile Conor got to hold his 145 belt for 3 fights out of the division just for a photoshoot holding up 2 belts he never defended once.
Before that, the OG double champs (Randy and BJ) both used the same method:
be a big name,
skip the line,
and switch divisions for free title shots
Anderson- non native English speaker, never got a chance at double gold even though he had legit top 5 wins in 3 weight classes.and racked up double digit title defenses.
If it wasn't for double standards, there were be no standards at all.