Dunny why, but my company decided quite some time ago that single engine taxi out is only approved on the A330, not the A320. Both are flown by the same pilots in mixed fleet flying. Single engine taxi in however is approved and encouraged on all fleets. The concern might be about misjudged engine starts, the PM (which is the captain on FO sectors) being head down and so on. But then, we moved from a policy of doing flight control checks before starting to taxi and no checklists from the start of taxi until being parked to the normal airbus SOPs which have us quite a bit head down during critical phases anyway.