You can connect two rack mixers to one console. https://support.presonus.com/hc/en-us/articles/360031210932-Series-III-Connecting-the-StudioLive-64S-with-two-32Rs-using-AVB
The only plugins that can be put on the series III mixers are from the fat channel plugins. https://shop.presonus.com/Software/Software-Line--StudioLive-Series-III-Mixers/Software-Category--StudioLive-Series-III-Plug-ins/Brand--PreSonus