  We present a detailed phenomenological study of photoproduction of two
hadrons, both with high transverse momentum, in longitudinally polarized
lepton-nucleon collisions. We consistently include ``direct'' and ``resolved''
photon contributions and examine the sensitivity of the relevant spin
asymmetries to the gluon polarization in the nucleon and to the completely
unknown parton content of circularly polarized photons. Our results are
relevant for the COMPASS and HERMES fixed-target experiments as well as for a
possible future polarized lepton-proton collider like eRHIC at BNL. So far, all
studies are limited to the lowest order approximation of QCD.
