W-2 Entry and Refund Amount

When only one W-2 is entered (either my wife's OR mine), TurboTax estimates a refund of about $1000 (but slightly different values for each).  However, when both are entered at the same time, it estimates over $9000 due.  How is this possible??

 

I entered my W-2 and TurboTax was estimating a refund of around $1000.  I entered my wife's W-2 and it changed to us owing over $9000.  I double checked that I entered everything correctly.  I also removed my W-2, leaving only hers and it again showed a refund of around $1000 (but not the exact same amount).  If the data from each individual W-2 shows a refund, how can they combine to have a tax due?