You cannot bridge a source unit. In a sense, the source unit is already bridged to get the power that it has without a switching power supply (The source unit is BTL= bridged transformerless).
The only way you could run two channels to one speaker is if the ceiling speaker has a dual voice coil midbass and two tweeters, which are commonly found in hallways and bathroom applications.
You cannot run a 2-ohm load with a source unit.
You can run an 8-ohm load with a source unit with a reduction in power.
You can run two channels into a 4-ohm load with two channels into an 8-ohm load without issue on a source unit. The load does not have to be equal.
The source unit can have two occupied and two dormant channels without issue.