There is no way to select 2 overlays side by side, only one at a time. The above message is a bit confusing because it suggests that several layers can be displayed at the same time. Instead of the list of layers, these are screenshots, only images that are not active.
Garming Inreach provides point forecast at your location or at waypoints. This light satellite receiver works on a narrow band connection.
You get Windy with a good internet connection near the coasts, while at sea you would need a broadband satellite receiver. For example Iridium Certus enables this type of internet connection up to 700kbps (down rate), but the cost and access subscription for this device are tremendously high.
Garming Inreach will never be a broadband receiver. Even Iridium Go which allows to receive GRIB files and so to get a simplified overall weather picture on a connected laptop, does not provide a high speed internet connection necessary for Windy.
Or it would need to develop a specific and very basic app only for this device same as Earthmate do.
Windy does not make weather forecasts, but just visualizes weather models data from leading centrers like ECMWF, NOAA, DWD.... So Windy cannot fix what you consider incorrect.
Have you tried the different weather models that Windy offers in your country? Some could be more performing in your area than the others.