The reason you are receiving each other's text messages is because you are both using the same Apple ID for iMessage. To prevent this you need to use different IDs. (Note: you can still share the same ID for iTunes.) On one of the phones, go to Settings>Messages>Send & Receive, tap the ID, sign out, then sign back in with a separate ID.
To migrate your 3GS to a new iCloud account, start by saving any photo stream photos that you want to keep on the phone to your camera roll. To do this, open the my photo stream album, tap Edit, tap the photos, tap Share, tap Save to Camera Roll. Next, if you have any notes that you are syncing with iCloud that you want to keep, you'll need to email these to yourself so they can be recreated after moving to the new account.
Once this is done, go to Settings>iCloud, scroll to the bottom and tap Delete Account. (This will only delete the account from this phone, not from iCloud. The phone that will be keeping the account will not be effected by this.) When prompted about what to do with the iCloud data, choose Keep On My iPhone. Next, sign back in with a different Apple ID to create the new account, turn iCloud data syncing preferences and when prompted about merging with iCloud, choose Merge. This will upload the data to the new account.
Finally, if you have merged data in the two accounts that you want to separate you will have to go to icloud.com from your computer, sign into each iCloud account separately and manually delete the data you don't want from each account.