You need to log into Windows 7, suspend Bitlocker (no need to disable your TPM), then reboot your computer into Windows 7 again (and reenable Bitlocker if it doesn’t automatically do it). This will “reset” Bitlocker to understand your new dual-boot configuration. Once you’ve done this, you should not need to enter your recovery key the next time you boot into Windows 7.
One possible option, I certainly don’t know if it is feasible for you or not would be to unencrypt the the Windows 7 partition, then encrypt again.
I Know, it takes forever and isn’t a very attractive option but from what I understand, BitLocker is picky when it comes to user fiddling with disks after the fact.
The other thing I could suggest in hindsight would have been to disable BitLocker encryption temporarily while you installed Windows 8.