I use one logbook for all.
The only "good" reason I have come across for two logbooks was where somebody was doing some illegal flying (basically, flying RHS in an MCC jet but without a TR and without an SIC, and doing it outside the USA) and he put those hours into a special logbook
The fact that under FAA rules one can log time one could not log under JAA rules means little in practice because
almost no non-JAA time is allowable against any JAA license or rating anyway. The 100hr PPL conversion route is the one exception I can think of. And in any case if you were to utilise any past hours towards something, the person doing the logbook examination will check all applicable entries.