When I installed uc_inventory_api, the only module that showed up on the admin/build/modules page was "Inventory API". "Simple Stock Levels" didn't appear in the list. I enabled "Inventory API" and it worked fine after I went to its configuration page and chose the stock manager. So it appears both modules did get installed, even though only one showed up in the list of modules.
The problem came when I tried to uninstall these modules. Since "Simple Stock Levels" wasn't on the list, I couldn't uncheck it to uninstall it. I was able to uninstall "Inventory API" just fine, but "Simple Stock Levels" didn't get uninstalled at the same time - visiting a product page and choosing the "Edit" tab would cause an error complaining about missing uc_inventory_api functions and the page would fail to display. So it seems "Simple Stock Levels" was still trying to do its thing, but now the "Inventory API" dependency was missing.
I don't understand enough (yet) about the internals of Drupal, so I can't say how to properly structure these two modules. However, as a hack, I eventually got Simple Stock Levels to show up in the list of modules by moving the install directory from site/all/modules/uc_inventory_api/managers/uc_stocklevels to site/all/modules/uc_stocklevels. Then it was a simple matter to uninstall both modules and get everything working again without inventory. Using uc_stocklevels_api version 1.7 and Ubercart alpha 8, Drupal 5.3