2 UK Passports, Permanent residency and Multi Re-entry

Advice please.

Because of frequent journeys to Cambodia and Laos, I have 2 UK passports.

One containing my re-entry permit (valid until Jan 2017) is almost full..

The 2nd has several pages left.

Can I travel out of Thailand with the passport without the re-entry permit, and show the 2nd passport containing the re-entry permit. I will of course produce my PR book, which also contains a re-entry permit and this will be stamped both when I leave and return.