Best to ask the existing S92 flights if they need 2 ac per flight or could they manage with one plus a floating spare. If you go for a spare option then you also need to generate a floating crew to position it when required, assuming your crews are pared down to the minimum required to comply with the EU working time directive.
I can see that scheduled servicing is an easy thing to factor in but not having a full time second ac leaves you open to missing availability KPIs in the event of a non-scheduled problem.