Hmm.
The thing about having 2 harmonisers, is the following questions:
1. will your sound become mush through the usage of 2 harmonisers?
2. what kind of sound, if not mush, would result from hooking 2 harmonisers together?
I can think of the following scenarios:
A. If Harmoniser 1 (H1) has a stereo output, just by using one harmoniser one can duplicate a 'twin lead' sound. Harmoniser 2 (H2) might be able to create a chorus effect, provided it has a detune function.
B. H1 is used to create a lower harmony note, with H2 using the DRY original, non harmonised signal to create an upper harmony note. When connected back into a stereo input (e.g. DD6) it will result in a 'triple' harmony sound.
C. H1 has an octave function, with wet signal going one octave up. H2 uses Dry signal from H1 to harmonise, thereby resulting in a 'twin lead plus one octave up' sound.
somehow there are many more possibilities... provided the harmonier pedals have stereo INPUTS, because most of them only have mono inputs...
However, one must bear in mind that the boss pedals require one's playing to be very tight, without any extra string noise because their tracking isn't exactly awesome... so if you intend to hook up two such harmonisers, bear in mind that MIGHT result in a significant delay between the (harmonised) H2 note and the dry H1 note.
It might sound thick, it might sound mushy... Up to you to go try it out!