I'm assuming your question was directed towards me. There is no such thing as an "assistant" captain. It's a common mistake made by pretty much every fan of the game. Though there are larger symbolic purposes behind the C and A's worn by hockey players, all the C really dictates is that player is the only one that can discuss calls with officials (though they are quite lenient in enforcing the rule). If for some reason the team's captain is unavailable one of the alternate captains fills in for him.
Edit: Here's the section on captains taken from the NHL Rulebook, http://www.nhl.com/rules/rule14.html
Rule 14 Captain of Team
1. One Captain shall be appointed by each Team, and he alone shall have the privilege of discussing with the Referee any questions relating to interpretation of rules which may arise during the progress of a game. He shall wear the letter "C", approximately three inches (3") in height and in contrasting color, in a conspicuous position on the front of his sweater.
In addition, if the permanent Captain is not on the ice, Alternate Captains (not more than two) shall be accorded the privileges of the Captain. Alternate Captains shall wear the letter "A" approximately three inches (3") in height and in contrasting color, in a conspicuous position on the front of their sweaters.
(NOTE) Only when the captain is not in uniform, the Coach shall have the right to designate three Alternate Captains. This must be done prior to the start of the game.
2. The Referee and Official Scorer shall be advised prior to the start of each game, the name of the Captain and the Alternate Captains of both Teams.
3. Only the Captain, when invited to do so by the Referee, shall have the privilege of discussing any point relating to the interpretation of rules. Any Captain or player who comes off the bench and makes any protest or intervention with the Officials for any purpose shall be assessed a minor penalty for unsportsmanlike conduct under Rule 41(b) - Abuse of Officials. Should this protest continue, he may be assessed a misconduct penalty, and if it further continues, a game misconduct penalty shall be warranted.
A complaint about a penalty is NOT a matter "relating to the interpretation of the rules" and a minor penalty shall be imposed against any Captain or other player making such a complaint.
4. No playing Coach or playing Manager or goalkeeper shall be permitted to act as Captain or Alternate Captain.