buddy system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An arrangement in which persons are paired, as for mutual safety or assistance.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The practice of working or travelling together with at least one other person, especially when undertaking something potentially risky or hazardous.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a cooperative practice of pairing two or more people together for mutual assistance or safety (especially in recreational swimming)