Yes - keep them out of line of sight at all times.
I've got their enclosures on the same wall where the snake is below the frog and they can't see each other. Which has been great. There's not been any problems except if I forget to scrub down after handling or cleaning the snake, and then go to change water for the frog. If the frog smells the snake she gets a bit wary for a while, but aside from that she's never shown any signs of stress.
Well, today I let the snake out to explore for a bit while I cleaned in the critter room. All of a sudden there is water splashing everywhere in the frog's enclosure and I turn around to see her frantically clawing at the screen lid with she has never done before. I didn't even think about it when I let the snake out- she saw the snake on the ground under her tank and completely panicked.She's okay, she only bumped her nose - which she does a lot chasing after crickets anyway - and as soon as I covered over her tank with a blanket she calmed right down.
From now on her tank will be covered before snake has any out of tank time, and we are planning on moving the frog out of there later this summer once her new tank is ready so that should permanently solve the problem.