One advantage of doing them all at the same time is that you catch all the crossover questions.
Otherwise, if you are going to split, the subjects that group best together for this purpose are:
HPL
AGK
Met (shares the atmosphere with HPL)
POF
Performance (includes POF)
Operational procedures (includes performance regs)
Instruments (includes compasses and INS, so convergency knowledge required)
Navigation (includes compasses)
Radio Nav (includes radio propagation)
IFR & VFR Comms (as above)
Law (includes radio failure in controlled airspace, and should be done before Flight Planning)
Flight Planning
Mass & Balance
Our order is slightly different to that:
HPL
POF
AGK
Instruments
Law
Ops
Comms
RNAV
Met
Navigation
FPM
Performance
M & B
At the moment!
Phil