The attainment program shall provide that upon application of the owner or operator of a stationary source, the commission may approve as a revision to the state implementation plan a proposal to meet the applicable requirements of the state implementation plan for a given air pollutant for two or more facilities or operations within such source through a combination of different requirements which separately may be more or less stringent than the applicable requirements of the state implementation plan; except that the total emissions from such facilities or operations shall not exceed the total emissions allowed by the applicable requirements of the state implementation plan.
C.R.S. § 25-7-305