  We construct martingale observables for systems of multiple SLE curves by
applying screening techniques within the CFT framework recently developed by
Kang and Makarov, extended to admit multiple SLEs. We illustrate this approach
by rigorously establishing explicit expressions for the Green's function and
Schramm's formula in the case of two curves growing towards infinity. In the
special case when the curves are `fused' and start at the same point, some of
the formulas we prove were predicted in the physics literature.
