Access to the outboard sides of the engines is as others have described--- you crawl around them. This can be easy, not so easy, or hard depending on the installation and other equipment in the engine space.
While a single engine boat generally has good access to both sides of the engine there can be advantages to having two engines that make the more difficult access to the outside of the engines worth it.
The single vs twin decision is based on things like economics, access, cruising area,and personal preference. I've run singles and have no particular predjudice against them. But having run a twin for the last 14 years we would never buy a single engine boat were we in the market for another one. Other people feel the same about singles.
I believe one can accomplish anything in a single that can be accomplished in a twin except come home under power if power is lost in a way that cannot be repaired on the spot, which happens. But in terms of maneuvering, anything that can be accomplished in a twin can be accomplished in a single even without a thruster. The techniques will be different but the end results will be the same.