so you want to put the content of iPad a onto iPad b and vice versa?
This is presuming you sync and backup to a computer. Sync each device and back it up. Then go into the settings on ipad a and 'erase all content and settings'. (in the settings/general menu)
then hook iPad a up to the computer you sync iPad b to and go through the setup procedure, When you see the option to 'set up as new or restore from a backup' choose the latter and choose the backup you just made of iPad b.
Do the same thing with the other ipad (just reverse it, use the backup of iPad a)
that should move the content of A onto B and vice versa