I don’t see the “separate” mobile menu you’ve mentioned.
Perhaps you’re regarding to “site-navigation” and “generate-slideout-menu”?
Those 2 are indeed separate menus within the DOM structure but they only show on different viewports. They don’t overlap or show at the same viewport. “site-navigation” only shows on desktop(or tablet depending on break point and “generate-slideout-menu” only appears depending on how you’ve set it on the customizer.
As for the slideout not working(perhaps you’ve notice this already). Your site seems to be using AMP.
You can try temporarily disabling AMP.
Off-canvas menus are known to not work well (yet) with AMP-enabled sites.