differential equations with few variables in Comsol 4.2

Hi for all!
I'm trying to simulate the heat and mass transfer process in wood during drying.
I need to investigate the distribution of temperature T and moisture content U in the wood during the drying process in air.
I have a set of equations that describe this process:

pCq*dT/dt = grad[ (X1+X2)grad(T) + X3grad(U) ]
pCm*dU/dt = grad[ (X4)grad(U) + X5grad(T) ]

U = f(t) - moisture content
T = f(t) -temperature
t - time
X2,X3 = f(T,U)
X1,X4,X5 - coefficients


I tried to solve it with PDE, but I don`t understand how I can introduce second variable U into eguation?
How this can be implemented in Comsol 4.2?

p.s. I should be grateful for your help!