Mogs, I have no experience of septic tanks since we had one in the UK, which had to be pumped out because the pipework became blocked with hanky type tissues. These had occasionally been used instead of toilet tissue, as we were unaware that they would not disintegrate sufficiently well.
If you Google "septic tank" you will see a wealth of information and diagrams, which should give you some idea of how it works, and possible problems.
It seems likely that the passage or pipe joining the two chambers is blocked, which would account for the difference in the two levels.
I think that "giving it a stir" may just add to your problems, by dislodging parts of the scum, which might contribute further to the blockage.
On some great and glorious day the plain folks of the land will reach their heart's desire at last, and the White House will be adorned by a downright moron.
H. L. Mencken 1880 - 1956
Some may not like his views, but what a prediction!