Are you both sure that it wouldn't be subjects then?
The primary motion and the 13 alternate motions are the subjects of the decision.
Or do you think subject is already okay?
"The primary motion, together with 13 alternative motions, is/are the subject of the decision."
Which is correct, is or are?
The subject of the sentence is motion; therefore, you have to use the verb in the singular, i.e. is
The primary motion, together with 13 alternative motions, is the subject of the decision.
If this is a response to my post: I agree in the context but I am simply responding to Nightmare85's post.
'together with', 'as well as', etc. are pseudo-coordinators. Pseudo comes from the fact (I think) that the conjoins affect the number of the verb according to the number of the first conjoin. If the two conjoins are singular, the verb will be singular. When the first conjoin is plural, the verb will be plural too.