I came across this piece where each hand utilizes a different key signature. Additionally, the key signature for the LH has the accidentals in the correct sequence but an octave lower than normally written. All this occurs in a parallel which appears should be reduced.
First question: Does the unusual key signature in the LH preclude reducing the parallel?
Second question: Should I follow MBC-15 6.5.1-2 Unusual Combined Key Signatures for both RH and LH since the RH is in a standard key signature (CM)?
It's my opinion that the different key signatures would prevent us from being able to combine the hands and reducing the parallel. Imagine how hard that would be to remember which hand is playing and which notes are sharped vs which are natural.
I believe following the method in 6.5.1-1 would be appropriate. And since the RH has no accidentals in the signature, you could just list the left hand. I don't think you need to do anything different that listing the usual 5# signature after the LH sign, even though the F# is on the first space of the treble staff, they are still in the usual order.
Hope that helps. And as always, if you feel like more clarity is needed, you can always add a sentence on your TN page about being aware that the hands may have different key signatures.