Behaviours and Triggers - xamarin

Question

I have a simple login form with two fields username and pass. Using behaviours I implemented email validation (not empty, valid email) and pass validation (not empty)
<StackLayout Padding="30" VerticalOptions="CenterAndExpand" Spacing="15">
<Entry Text="{Binding Email}" Keyboard="Email" IsEnabled="{Binding IsNotBusy}" Placeholder="Email address">
<Entry.Behaviors>
<local:EntryValidatorBehavior IsCheckEmail="true" ValidateFromStart="false" IsCheckEmpty="true" x:Name="emailValidator" />
</Entry.Behaviors>
</Entry>
<Label Text="{Binding Source={x:Reference emailValidator}, Path=Message}" />
<Entry Text="{Binding Password}" IsPassword="true" IsEnabled="{Binding IsNotBusy}" Placeholder="Password">
<Entry.Behaviors>
<local:EntryValidatorBehavior ValidateFromStart="false" IsCheckEmpty="true" x:Name="passwordValidator">
</local:EntryValidatorBehavior>
</Entry.Behaviors>
</Entry>
<Label Text="{Binding Source={x:Reference passwordValidator}, Path=Message}" />
<StackLayout Orientation="Horizontal">
<Button Text="Forgot Password?" Command="{Binding ForgotButton}" TextColor="White" HeightRequest="40" VerticalOptions="End">
</Button>
<Button HorizontalOptions="EndAndExpand" Text="Sign In" Command="{Binding LoginButton}" TextColor="White" BackgroundColor="#f50057" HeightRequest="40" WidthRequest="100">
</Button>
</StackLayout>
</StackLayout>
This works perfectly, but at the bottom of the form I have a Login button which should be hidden as long as both email and password are not valid.
How can this be implemented? I was thinking by using Triggers
<Button HorizontalOptions="EndAndExpand" Text="Sign In" Command="{Binding LoginButton}" TextColor="White" BackgroundColor="#f50057" HeightRequest="40" WidthRequest="100" IsVisible="false">
<Button.Style>
<Style TargetType="Button">
<Style.Triggers>
<MultiTrigger TargetType="Button">
<MultiTrigger.Conditions>
<BindingCondition Binding="{Binding Source={x:Reference emailValidator}, Path=IsValid}" Value="True" />
<BindingCondition Binding="{Binding Source={x:Reference passwordValidator}, Path=Text.IsValid}" Value="True" />
</MultiTrigger.Conditions>
<Setter Property="IsVisible" Value="True" />
</MultiTrigger>
</Style.Triggers>
</Style>
</Button.Style>
</Button>
but this doesn't work.
Thanks in advance!