Someone recently was asking about land border crossing with different passports but comments are closed already. Actually it is possible. I did this trick in April or May. I left In Mukdakhan, then got Lao visa in clear passport (they just asked to show an old one), then came back in Mukdakhan in a few hours since embassy had some long holidays. Hope that helps.
TLDR : Answer Summary
A user shares their experience of crossing from Thailand to Laos and back using their old passport to obtain a Lao visa. They clarify that this method involves showing the old passport to verify exit stamps. Comments discuss the nuances between using different passports of the same nationality versus switching nationalities.