Single engine is the lesser of two evils. The old saw of your needing two if one craps out is mute today with the reliability factor present in newer motors. Besides the most problems encountered revolve around bad gas & filters. This would just double your problems.
Lastly think long & hard about a good deal when it comes to used engines. You may be a paragon of virtue when it comes to motor maintainance, but; was the guy your buying from?
Bob leonard