Using series P-Channel MOSFETs for high-side switching
Without more information, it's hard to say, but I suggest that you may be using ideal FETs in your simulation. Ideal FETs will have no leakage current, so the junction between the two will be floating, so the top FET, even though turned on, draws no current and its model doesn't work well.

I suggest two possibilities. The first is the most obvious - replace the FETs with actual, real part numbers. These will have leakage currents modeled which will allow more realistic operation. 

The second is like unto the first. Place a fairly high resistance, say 1 to 10 megohms, between drain and source of each FET.