I'm not sure I get what you are asking but the second one doesn't work as a construction the way you have formatted it. The sentence structure we currently have been shown in the course is slightly different to one which would deal with multiple prepositions, which is partly your issue there.
The phrase còmhla ri chèile is partly idiomatic I guess. Còmhla ri is the bit that means together, or along with. Cèile means partner (can also be used in the romantic sense), or other. So còmhla ri chèile means something along the lines of "together with each other".
So to part answer your question (I think), còmhla ri chèile ri Seumas doesn't work as a construction because of the same reason it doesn't work in English. It translates as "together with each other with Seumas"
Tha sinn còmhla ri Seumas would be the phrase as it literally means "we are together with Seumas". You use còmhla ri when dealing with people, and le when dealing with other things. Don't make the mistake of saying "tha sinn le Seumas" because whilst that does say "we are with Seumas" that implies it's a very "hand's on" experience.