Upside of having two characters?

[Krr]

Weekly lockouts is the big one, and one of the reasons I've seen raiding buddies with a lot of free time to burn make and level second characters.

[kuikui]

The only reason to play more than one class is simply to break up the monotony of one job. Once you get into the post 50 grind for virtually everything to progress, you'll begin to see just how tedious and time consuming it becomes to keep 2 or more characters on par.

I started out doing it on smn, pld, and drg. Then dropped to smn and pld. And now pretty much just keeping smn completely up to date and keeping pld up just enough to be competitive in order to get instant content queues.

[Mishini_Dracoto]

The only reason to play more than one class is simply to break up the monotony of one job. Once you get into the post 50 grind for virtually everything to progress, you'll begin to see just how tedious and time consuming it becomes to keep 2 or more characters on par.

I started out doing it on smn, pld, and drg. Then dropped to smn and pld. And now pretty much just keeping smn completely up to date and keeping pld up just enough to be competitive in order to get instant content queues.

Are we playing the same game? on ONE Character, you can play EVERY class.

Is there any great benefits to having two characters?

Some, which are fairly minor. it depends on how much time you have to invest. If you don't like the way your character looks, you can always buy Fantasia and change. I would recommend this instead of re rolling except in certain circumstances ; such as one character is capped ; you are bored and not enjoying it / want to start fresh / want to start on a new server for some reason.

[Alistaire]

What exactly is the downside?

Also everything that people have said here but also maps, having your dex/int jobs on an elezen and str/mnd jobs on a roegadyn, that kind of thing. Not a huge boost but there's not exactly a huge detriment to running 2 characters.

[Sadana]

It's fun! (I have 13.)(maybe 14.)
You could always use the new character to refamiliarize yourself with the game and swap back later if you want to.

[Rikkustrife]

more ventures, can get twice as many tomestones

Plus it feels different playing different characters so it has some variety , and there is no reason not to

[Mattlex]

If you only have Warrior and Black mage levelled it is worth just starting a new class on the same character. There are benefits to having a second character but you will have to do a lot of the main scenario and GC stuff again to be able to ride mounts etc etc, I would only reccomend starting a whole new character if you plan on doing all those work arounds for lockouts etc people mentioned. Otherwise just start a new class, I found Archanist a fun class to play but you have many options without starting a new character.

[Mini_Me]

Not wanting your lalafell to look like 'Ted' when tanking or kicking somethings ass. I prefer a Rogayden for tanking, melee just doesnt fit with being a lalafell in my opinion.

[Riastrad]

This is a strange thing to do/ say but... I have 6 alts all with level 50's and relics for main jobs.
After I cap tomes on my main character for week (usually Wednesday night) I play another till I grow bored with them or cap tomes. It is incredibly easy to do so because of frontlines (not that I'm complaining). Granted, most of these characters are from 1.0 and had a head start on levels.

Pro tip: Place them in the same FC so that you can share items between them along with Gil. If your main ever gets low the quest rewards from the storyline will come in handy.

[SummonerSenah]

I liked seeing the different starts to the main story based on where you started. On my main I started in Limsa and on my alt I started in Gridinia. Also, one is a Miqo'te and one is a Hellsguard Roe, so they look very different. It was fun playing with the character creator. The biggest advantage to me personally was being an arcanist on both and taking one down the summoner route with full 30 in intelligence, and taking the other down the scholar route with 30 in mind.