yes could be an issue . Because i guess they will question how you get in the neutral border and again i think you will show them both passport and ask for stamps on your french one . ( Then you let me know how it worked π)
Is there no e-visa need for Cambodia ?
Because if you need the e-visa then you explain that you applied for Cambodia visa based on french passport π€
Im curios me too on any answers on your particularly questions go out of thailand with "A" passport entry cambodia by land with "B" passport then get back to Thailand with "B" passport βΊοΈ ...