ManyPagesAvalonia/AvaloniaApplication10/ViewModels/MainWindowViewModel.cs

using Avalonia.Controls;
using ReactiveUI;

namespace AvaloniaApplication10.ViewModels
{
    public class MainWindowViewModel : ViewModelBase
    {
        UserControl _us = new Page1();

        public UserControl Us {
            get => _us;
            set => this.RaiseAndSetIfChanged(ref _us, value); 
        }
        

         Page1ViewModel _page1VM = new Page1ViewModel();

        public Page1ViewModel Page1VM 
        { 
            get => _page1VM; 
            set => _page1VM = value; 
        }
        public Page2ViewModel Page2VM 
        { 
            get => page2VM; 
            set => page2VM = value; 
        }

        Page2ViewModel page2VM = new Page2ViewModel();

        public void ToPage2()
        {
            Us = new Page2();
        }

        public void ToPage1()
        {
            Us = new Page1();
        }
    }
}