Database with replication and mirroring stuck in LOG_BACKUP after log file shrink
I wonder if your operations generated some gaps in the transaction log which may be preventing the virtual log file from looping back around to the front of the physical log file. Conceptually this is what should be happening within your database when the end of the physical file is reached:



If DBCC LOGINFO returns records indicating the VLFs at the beginning of the physical log file have a status of 2, these VLFs need to be cleared, which in Full Recovery mode, means a TLog backup must be taken.  I believe this situation can also cause the log_reuse_wait_desc column to show LOG BACKUP as well. To fix this, you must force the logical header back toward the beginning of the physical file to clear out these status 2 VLFs (and hopefully get your log_reuse_wait_desc column back to showing NOTHING).  To do this, perform the following steps (which may need to done on both DB1 and DB2 databases):


Perform a Log Backup (e.g. BACKUP LOG [DBNAME]...) against the database
Perform a DBCC SHRINKFILE operation on the transaction log file specifying 1 MB as the target size
Perform another Log Backup against the database
Perform another DBCC SHRINKFILE operation on the transaction log file specifying 1 MB as the target size
Manually increase the size of the log file, incrementally per Kimberly Tripp's article, to grow the TLog back to an appropriate size.


So what's happening here?


The first Log Backup simply generates a backup of the completed transactions that have been flushed to disk and deletes inactive virtual log files from the logical transaction log, freeing space for reuse up to the logical log header.
The first DBCC Shrinkfile operation shrinks the physical TLOG down to where the current logical log header is located, freeing as much space as possible.
The second Log Backup operation is now able to loop back around within the logical log and backup/clear any residual inactive VLFs located before the logical log header.  These were probably never flushed in the past for whatever reason, but should be cleared at this point.  This step is critical to fixing your issue and sometimes this backup can take a bit of time depending on how much data needs to be written down to disk.
The second DBCC SHRINKFILE operation moves the logical log header to the front of the physical log file, eliminates excessive VLFs that are likely there now, and reduces the TLOG file back down to a very small size, in this case 1MB.  It is imperative to manually grow the file in step 5 so you don't run into VLF issues again down the road
Manually increasing the size of the tlog will allow you to control the size and number of VLFs within your physical log file.  This is an opportune time to configure this properly, so take advantage.


One final note on the process.  If your tlog is highly active, you may need to perform this a few times or during a window of reduced activity.  High activity may result in another errant auto-growth event occurring before you execute the second log backup.  This basically turns what you thought was step 3 back into step 1.  Generally Steps 1 and 2 go by quickly, and step 3 takes the longest to complete.  Make sure to follow-through with steps 4 and 5.