Here's the scenario. Person has 2 UK passports, both unexpired, but 1 is filling up so she wanted to transfer the long-term Thai re-entry visa the new passport (since it is the Thai stamps that fill up the passports). BKK one-stop visa officer said "cannot" - must cancel 1 passport before transferring, or travel with 2 passports. Clearly travelling with 2 passports defeats the objective of having 2 passports for the frequent traveller (i.e. you leave 1 behind in an embassy while a visa is being processed). Immigration officer said this is internal policy (i.e. no written document saying so). Anyone had similar experience ? Thanks.

Visas are never, ever, transferred but suspect you are talking about an extension of stay stamp/re-entry permit? The requirement for that transfer is an Embassy letter requesting it for a lost or canceled passport. It is never done between active passports that I have ever seen reported.

Just use the full one for entering and exiting Thailand and the other when arriving/departing other counties.