Are there multicellular isogamous species?
Our knowledge about the sexual transmission of mitochondria and
  plastids (hereafter organelles) in isogamous eukaryotes comes mostly
  from studies of the yeast Saccharomyces cerevisiae and the green alga
  Chlamydomonas reinhardtii which are both unicellular species. To
  investigate organelle inheritance in a multicellular organism with
  morphological isogamy, we studied the filamentous brown alga
  Ectocarpus siliculosus, in which each gamete contributes one plastid
  and at least one mitochondrion to the zygote. ...



2004 - Inheritance of organelles in artificial hybrids of the isogamous multicellular chromist alga Ectocarpus siliculosus (Phaeophyceae)


So it is possible, but it seems like very rare.