I am not well versed in Zygopetalum's.
But, per the IOSPE [
Internet Orchid Species Photo Encyclopedia ], they are separate and distinct species.
With that said, as usual, there seems to have been a few naming disputes in the history of these Zygopetalum's:
Zygopetalum intermedium Lodd. ex Lindl. 1844
!Zygopetalum mackaii Hooker 1827
[ The ! = type species for Zygopetalums ]
~Zygopetalum mackaii Paxt not Hkr. - See Zygopetalum intermedium Lodd. ex Lindl. 1844
~Zygopetalum mackaii var. intermedium Mutel 1842 - See Zygopetalum intermedium Lodd. ex Lindl. 1844