There's no real integration between the two IDEs. As far as I know, Rider detects file changes the same way as Visual Studio, so if you really want to build the UI with Visual Studio and do the rest using Rider, you can do it with quite a smooth workflow.
Also, it's worth it to learn to write WPF UI code by hand. Nowadays, I mostly write my UI code by hand instead of using the designer. With Windows Forms applications, it's different since editing the *.designer.cs
files can be a bit of a pain by hand. It's doable though.
Update (04/2018): Rider 2018.1 now includes a XAML preview tool for WPF projects. It's still under active development and the preview window is non-interactive, but it's certainly better than nothing.