  We report on the magnetic, thermodynamic and optical properties of the
quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been
discussed as spin-Peierls compounds. The observed deviations from canonical
spin-Peierls behavior, e.g. the existence of two distinct phase transitions,
have been attributed previously to strong orbital fluctuations. This can be
ruled out by our optical data of the orbital excitations. We show that the
frustration of the interchain interactions in the bilayer structure gives rise
to incommensurate order with a subsequent lock-in transition to a commensurate
dimerized state. In this way, a single driving force, the spin-Peierls
mechanism, induces two separate transitions.
