Morning,
Weka only supports single class attributes. However, untested, I think you may be able to work around this. Try the following, I'm interested as to what kind of accuracy you get.
eg data.
BloodIron real
BloodSuger real
BloodColor real
Diabetes bool class
Lifespan real class
Using the data above, train to solve only the 'Diabetes' class. Once you have the variables enter them into your dataset and use them to solve your second 'LifeSpan' class attribute.
To increase accuracy solve the class variable with the highest entropy first.