@Steven ******
I'm a UK immigration officer, and have worked as a Liaison Officer to Thailand. You won't have a problem. It's perfectly normal for a passenger to carry multiple passports like this, something we routinely encounter every day.
It's extremely unlikely that they'll ask to see your boarding pass upon arrival, but if they do it'll be absolutely fine if you show both passports and explain the different names. They won't even blink; I can understand your concern, but I can assure you this is a non-issue, you've nothing to worry about.
But, in any case, the chances of them even asking to see the boarding pass are vanishingly small, so you'll probably only need to show them one passport.