Cannot define 2 fluids simultaneously?

Bengonia.Boom8

Hi all, I am going to simulate a fuel-oil cooler, and I find that I cannot define 2 fluids types in this analysis. For ex. I have 3 domains--solid(aluminum),fluid1(fuel),fluid2(oil), but I just cannot define two different fluid types in this analysis ! When I setup fluid1 as fuel1 then the fluid2 will be changed to fuel1, no matter what has been assigned to fluid2, and vice versa. Thanks in advance!