Call an xaml element from cs file

I created a button in my main file (SurfaceWindow1.xaml) and used this code in the .cs file to give it a function.

void changeMapStyleHy(object sender, EventArgs e)
{
SurfaceMap.MapStyle = InfoStrat.VE.VEMapStyle.Hybrid;
}

Works fine, no problems there.

I created an extra xaml file which loads when I place a tag on the screen (I'm developing on a Surface Table). This works fine too. The 2nd xaml is loaded and showed up.

Now, when I create a button in this second xaml file, I can't 'call' "SurfaceMap" since it's in my main xaml file. How can I refer to this element?

Thanks in advance!