Albert Barnes Commentary Acts 21:33

"Then the chief captain came near, and laid hold on him, and commanded him to be bound with two chains; and inquired who he was, and what he had done." — Acts 21:33 (ASV)

To be bound with two chains. To show the enraged multitude that he did not intend to rescue anyone from justice, but to keep the peace. Paul's being thus bound would convince them of his determination that justice should be done in the case. Probably he was bound between two soldiers—his right arm to the left arm of the one, and his left arm to the right arm of the other. (See Barnes on Acts 12:6).

Or, if his hands and feet were bound, it is evident that it was done in such a way that he was still able to walk (Acts 21:37–38). This was in accordance with the prediction of Agabus (Acts 21:11).