It's possible to open two windows side by side on an iPad or on an Android phone. Unfortunately, it's not currently possible on an iPhone.
On an iPad, open Gospel Library. Then pull upward from the bottom of the screen to show the iPad dock. Then long-press and drag Gospel Library from the dock to one of the sides of the screen. For the use case of opening the same content in two different languages, there's an easier way: Open Gospel Library to content you want to study, then tap the overflow menu in the top right, tap Languages, and long-press the other language. Choose "Open in New Window."
For Android, start by opening two Gospel Library screens (one in each language). You should see both screens in the app switcher, as if they're two different apps. From there, instructions vary by device – here are some instructions for a Google Pixel phone:
https://support.google.com/pixelphone/answer/7444033?hl=en