<telerik:RadSlideView x:Name="slideview" AutomationId="slideView" InteractionMode="Pan" >
    <Grid>
        <telerik:RadTreeView x:Name="treeView"
                    AutomationId="treeView"
                    ItemsSource="{Binding Items}">
            <telerik:TreeViewDescriptor DisplayMemberPath="Name"
                            ItemsSourcePath="Children" 
                            TargetType="{x:Type local:Item}" />

            
        </telerik:RadTreeView>
    </Grid>
    <Grid>
        <Button Text="Item 1" HorizontalOptions="Center" VerticalOptions="Center" />
        
    </Grid>
    
    
        <telerik:RadSlideView.Behaviors>
                <local:SimultaneousTouchBehavior />
            </telerik:RadSlideView.Behaviors>
</telerik:RadSlideView>

public partial class SimultaneousTouchBehavior : Behavior<RadSlideView>
{
    private RadSlideView? slideView;

    protected override void OnAttachedTo(RadSlideView bindable)
    {
        base.OnAttachedTo(bindable);
        this.slideView = bindable;

        this.slideView.HandlerChanged += this.OnHandlerChanged;
    }

    protected override void OnDetachingFrom(RadSlideView bindable)
    {
        base.OnDetachingFrom(bindable);

        if (this.slideView != null)
        {
            this.slideView.HandlerChanged -= this.OnHandlerChanged;
            this.slideView = null;
        }
    }

    private void OnHandlerChanged(object? sender, EventArgs e)
    {
#if IOS
        if (this.slideView?.Handler?.PlatformView is UIKit.UIView platformView)
        {
            if(platformView != null)
            {
                foreach(var item in platformView.GestureRecognizers)
                {
                    item.ShouldRecognizeSimultaneously += (item, otherGestureRecognizer) => true;
                }
            }
        }
#endif
    }
}