I think Columbo or someone else in the Homicide department must have mentioned or discussed the idea of accomplices being involved, but the fact that Stone had Hayward's clothes on when he was murdered, leads Columbo to the candidate's door anyway.
If there were actually more than one person involved in the murder, then Hayward wouldn't need Stone to have to go through all the clothes switching. They could have killed Stone at his own home giving Hayward an air-tight, iron-clad alibi.
There is actually no way Hayward could have beaten Stone to the beach house. It is a flaw in the plot, that's why they don't show how he does it. Only the staunchest Columbo fans can explain or defend this.
However, irrespective of the flaws, this ending is as fine as 'Playback' or 'Suitable For Framing'. It's literally a killer ending, compounded by the suddenness of the episode credits again.