That thing where he comments about the size of the wizard's fireball should be met with a response like "Oh, how about this? *blam* It looks bigger from the inside, doesn't it?" .
LOL! I have to admit, that was my first thought, as wee. But, we have to keep in mind that the player in question is also prone to being pissy and bitter if something happens to his characters that he doesn't like.
Alright, here's my advice for dealing with your situation.
1) Don't allow multiple characters to be played by any one player. Make this a rule for everybody, across the table. All cohorts and henchman are to be played by the DM. Listening to this dude monopolize the game, roleplaying between his two characters must be annoying as hell for everyone involved.
2) Take a strong role in character creation. Don't be afraid to say "the extraplanar traveller in the guise of a monk, doesn't fit my campaign direction. Let's try something else". In my own game, I find it frustrating when characters who have supposed to have known each other for years, act like they've just met. So, I'll tell them that they're friends, and that I expect them to play that way. XP bonuses when they do, give them reasons to continue.
3) Make horrible things happen to the "explorer" when he pushes the line. You need to be careful with this one, as you don't want to quell the characters abilities to be free thinking, and explorative, but, you need to keep control of the game.
4) Have an NPC here and there that will stand up to the "explorer" character, and get him back in line. It sounds to me like he is the dominate personality of the group, and he is able to manipulate characters and situations to go his way. If you show the other players that he can be dealt with verbally, with an NPC that has a little gusto, they will likely follow suit and stand up to his railroading.
5) Don't blame one, for the sheep like reactions of the bunch. A leader, good or bad, leads. If the other characters are following the "explorer" into certain death, then death it is.
Thats all I can think for now (actually, I'm just tired of typing), so I look forward to your replies.