Perhaps you might consider an IDIQ contract for the services that would be used by both projects. and issue task orders as needed.  The government does not enter into subcontracts.  If you decide to go with a single contract, the prime contractor may accomplish the work via a subcontract.  The government would not interfere with that contract between the prime and their subcontractor.  There is great flexibility in determining the best contract-type strategy.  Plese see FAR part 16 and FAR part 17.