Translation:Stephanus goes to the market with Corinna.
It's not a compound subject. "cum Corinna" is a prepositional phrase, and the subject of cum takes the ablative case. If it helps, Stephanus is the only word in the nominative case, so the subject is singular and the verb must be singular to agree with it.
Latin has case endings specifically because it's not tied to word order, outside of poetry. Stephanus goes to the market with Corinna AND Stephanus goes with Corinna to the market should be accepted.
stephanus (he) is going to the market, with Corrina.
stephanus and corrina (they) are going to the market.
the verb changes depending on if you use "cum" (with) or "et" (and).