The BetaFPV RC transmitter should already use a modern frequency hopping protocol (much like a phone) that allows multiple radios to be used without interfering with each other. One of the advantages of broadcasting in the GHz range is that they can send hundreds of data packets every second. Each one is sent at a different frequency, in a pseudo-random pattern that the receiver can follow. There are about 100 different frequencies in the band, so one packet in 100 may conflict with the second transmitter, but the receiver just ignores it and you don't notice the slight delay.
The FPV video is more old-fashioned. It uses a single frequency, and there's only room for 6-8 in the whole 4.8GHz band because they're much higher band-width signals. (Note that different manufacturers have split it in different ways but they're all using the same band) You have to pick a frequency manually. There are two ways to do this. Some (older) video transmitters have a button to step through the frequencies and LEDs to indicate what channel they're using.
Most VTXs these days can be configured by the flight controller, through a menu in the on-screen display. When you first power up, the OSD displays instructions for getting into the menu for a few seconds.
Throttle center, Yaw left, Pitch forward
Once you have done this, you will see the menu open up on the screen.
You can now navigate the menu using the right stick (pitch/roll) to enter, exit, and scroll items.
With the OSD menu open, navigate to Features > VTX SA. This will open all of the Smart Audio settings.
Within this menu, you can now set the video band, channel, and power option to your preferred setting.
To save your new settings, use the Set item near the bottom and navigate back to the main menu.
There are some pictures here: