Both of these summonings you can get by going to the Land of the Summoned Monsters. This is via a cave in the Underground that is on a little island just north of the Sealed cave and West of the Dwarven Castle. You can visit this place anytime during the game once you have the Falcon Airship and it has been fixed to fly over the magma.
The cave that leads to the land of summoned monsters has three levels with several hidden passages and guarded items. It can be a good place to level up as it is quite challenging. Once past the three levels, you come to the a city full of monsters. Note that if you wish to get Cecil's upgraded sword Excalibur, you have to at least arrive to here. You do not have to beat Asura or Levitan, but there is an item that is only found here in the Land of the Summoned monsters which you must have to complete that side quest.
Getting to the Land of Summoned Monsters
The Sylvan Cave is located in the North West corner of the Underground and like the Land of the Summoned Monsters you can only get there once you have the Falcon and it can fly over the magma. You do not actually have to fight the Sylphs to get them as a summon, you instead have to meet them, perform a task, and then return. While not an overly powerful summon, doing this side quest results in learning about the fate of one your former comrades.
To get the Odin summon, you must complete the Land of the Summoned Monsters Side quest. It is not clear if you need to just go to the land of summoned monsters or defeat Asura and or Leviatan, but it certainly helps if you do have Leviatan as a summon to defeat Odin. Once you have, return to Baron and go to the bottom floor of the right tower.
Bahamut is by far the best attack summon in the game and definitely worth getting. You can get Bahamut once you can control the Big Whale on the moon, so right before you start the journey to the Moon's core. The path is quite hard as you have to fight several Behemoth fights along the way, but you can pick a full set of good Samurai armor for either Kain or Cecil.