Turn on a multi-functional pressure cooker (such as Instant Pot(R)) and select Saute function. Heat olive oil; add couscous and mung beans. Cook and stir for 2 minutes. Add vegetable broth and 1 teaspoon salt. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 20 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions for 10 minutes and then quick-release the remaining pressure, about 5 minutes more. Unlock and remove lid. Fluff couscous with a fork. Let sit until cool, about 10 minutes.
Combine couscous, mung beans, cucumber, tomatoes, lemon juice, and cilantro in a bowl. Sprinkle with 1 teaspoon salt and pepper.