The OctoSPI does support dual-quad mode as well, so yes, it's possible to connect two flash chips in QPI mode, one of them using IO7-IO4 the other one IO3-IO0. If you have only one, set DQM to zero and select via FSEL whether you want to user the "upper half" or the "lower half". This works exactly in the same way as for the QSPI interfaces.
See e.g. Fig. 136 in RM0455 for H7A/B and the description of OCTOSPI_CR, bit 7, FSEL.
