So currently there’s no way to wear multiple or even two types hairs or any accessories (excluding hats) on Roblox mobile.
if you do try to change your avatar from Safari or Google Chrome, it’ll tell you to customise it from the mobile app.
This also goes for extended skin tones, there’s only a specific and small set to choose from unlike PC. Not to mention that you can only change your skin overall, instead of specific body parts like on PC.
mobile: (not the whole thing, a bit more at the bottom)