concatenate scalar Functions together to one VectorFunction

I'm solving a complex-valued problem for which there is no straighforward support in Dolfin, but where you have to take the real and imaginary part of your equations apart. This works well and gives me the real and imaginary part of the solution, phi_r and phi_i.
For plotting them out to a file, what I do is

f = File('phi.pvd')
f << phi_r
f << phi_i

which treats phi_r and phi_i as different (time) steps. Ideally, I'd write out phi_r and phi_i as one state, much like VectorFunctions are treated.
Is there a way to concatenate phi_r and phi_i?