suggest googling IRS Publication 523 and reading it!
proration is simply not an issue - see the publication
even if you bought the house today, got two tenants TODAY who lived with you for two years in your principle residence and at the end of the two years, you sold the house, you'd still be eligible for the $250,000 capital gains exclusion and there would be no proration.
As stated above, the only issue would be the depreciation recapture tax as you were presumably depreciating the property and reporting the tenant rent as income
