Hi, I see - this is in the C++ version 3 of the code. Please do mention which version of the code you are asking about since we're currently supporting two different langauges, as mentioned in this Guidelines post: http://itensor.org/support/2003/guidelines-for-asking-questions
I'll make a note to update the answer above soon with a Julia version answer.
The short answer is that, yes, it's also supported in Julia. All you have to do is make an array of Index objects which alternate between two different site types. In Julia a site type is denoted by a special Index tag type. So for example the array:
sites = [Index(2,"Site,S=1/2,n=1"), Index(2,"Site,Fermion,n=2"),Index(2,"Site,S=1/2,n=3"),Index(2,"Site,Fermion,n=4")]
would define a 4-site system with alternating S=1/2 and spinless-fermion sites. You can then use this sites array to define MPS; create an MPO with AutoMPO; etc. and our system is smart enough to figure out what operators to use etc. just by inspecting the Index tags ("S=1/2" versus "Fermion" in this case).