If you have two logbooks, then they should both have the same totals. That is to say - if you do a flight it should go in both logbooks.
The only reason for having two is that PIC is logged differently in the US. Basically they allow you to be PIC whilst receiving instruction post-pvt so you could log a flight as both DUAL and PIC. Under EASA it is one or the other.
You can get around this on most professional logbooks by using a spare column to log two types of PIC. If you only intend to get CPL/IR under EASA then you really don't need the FAA logbook - and vice versa. Ultimately a logbook is a record of your flying for (a) showing flight time for a certificate or licence (b) showing flight time for a job, (c) showing recency, or (d) personal gratification.
*So they all need to be in the same book*
Edit: it really doesn't matter if you log minutes or decimal