I used "stamp transfer" as descriptive term.
When he attends immigration for "stamp transfer" the first thing that is entered in new pp is a stamp that the officer fills in.
The replaced passport # , type of visa etc.
After that is done i can't see requirement to carry both passports.
Just the new one and print out of e-Visa should be sufficient.
Perhaps someone that has obtained new pp and has a DTV can post first hand experience.
OP, you may find this thread useful.
https://aseannow.com/topic/1335168-what-happens-when-i-renew-my-passport-before-my-dtv-expires/