Tell me if this does the trick for you.

Use your sonar device to show which one is which. The poem states as follows: "A frog that eats silver and a frog that eats lapis guard the king's floodgate one side apiece. Impress them each to open the gate. Lapis, lapis, silver, lapis, silver, release." So what you do is go to the stone frog click the switch twice, then the metal frog, click the switch once, stone frog, switch once, metal frog once. Hope it helps.