If cooling failure occurs for any reason, a redundant system will keep your cellar environment from being compromised. Redundancy can be a second cooling unit, or the home A/C system can be used. A backup generator is a good idea in case of power failure. You should have some kind of contingency plan in place in case of electrical or mechanical failure in the cellar.
Wine cellar cooling units require maintenance to perform at their optimum officiency. With a redundant system, one cooling unit can remain operational while maintenance is being performed on the other.
Sometimes it's a challenge to cool a large cellars that is in excess of 2000 cbft. with one powerful cooling unit, it's invaluable to have a redundant system with cooling units on opposite sides of the room.
Rather then using a larger cooling unit in your cellar, a redundant cooling system (2 smaller units) may provide you with a quieter operating system with a smaller footprint.