I have two iTunes accounts, how can I merge/manage them both ?

Sign into the one account and you can update and download with that account. If you need to use the other account go to Settings>Store>Apple ID - tap that ID and sign out and then sign in with the other account.

If you sync with iTunes or download content in there - you have to be signed into the corresponding account on the iPad and the computer.

You cannot merge two iTunes accounts so you just have to keep going back and forth between them. My advice would be to pick one for all future purchases and only use the other when you need to update apps or sync.

You cannot merge Apple IDs, so content in an iTunes account controlled by one Apple ID cannot be merged with another so that you can then abandon one Apple ID. But you can merge the content from multiple iTunes accounts into the same iTunes libraries on the same computer. You need to authorize the computer for the content. You do so by signing into an iTunes account in the iTunes app on the computer and then authrizing the computer for content from that account in the Store dropdown menu. That merged library can then be synced to multiple iOS devices. The first time you use content from one of the accounts, you will most likely be asked to sign into that account to authorize the use of the content from that account on that device.