My insurance rep said I could not have 2 auto insurance policies per household?

Is this true? There is a total of 5 vehicles in my household along with 4 drivers. There is 4 full coverage vehicles on one ins. policy, but we tried getting a separate insurance policy for the 5th vehicle(an old car) with just liability only. She told us we could not have 2 separate auto ins. policies in the same household and that we would have to add the 5th vehicle to the one policy we already have. I'm in the state of Texas by the way...