I’ve had more than two beetles at a time. Sometimes there will be three in a row on the same side of the island, but it doesn’t happen very often.
What I did on my island was remove all the flowers, all fruit/regular trees and the shrubs. Then remove the palm trees near the bottom of the beach to make it easier to go to each side without scaring any beetles. I go through the middle, up the left side then down to the middle again and up the right side. If no beetles appear after doing this for a few times, I go inside the hut and then back outside.
To increase the chances of a beetle appearing, make to scare away all other insects that spawn. Wharf Roaches and Tiger Beetles will still spawn.