how can i make a flow simulation study if i have two fluids (water &steam ) in the same vessel?

If you supply more information you may get a way to apprompximate your situation but as stated the answer is you can't.

2 fluids in the same passage or different.

Different > yes. Take a look at the heat exchanger example in the tutorials. And take care when using steam that you are in the limits of the model.

Same>no...but, you could get close by making your own fluid with the appropriate properties depending on what you need the fluid to do.