<ResourceDictionary>
    <OnPlatform x:TypeArguments="x:String" x:Key="HeavyFont">
        <On Platform="Android" Value="YourFont-Heavy.ttf#YourFont" />
        <On Platform="iOS" Value="OpenSans-Bold" />
    </OnPlatform>
    <OnPlatform x:TypeArguments="x:String" x:Key="RegularFont">
        <On Platform="Android" Value="YourFont-Regular.ttf#YourFont" />
        <On Platform="iOS" Value="OpenSans-Regular" />
    </OnPlatform>
</ResourceDictionary>

<StackLayout>
    <Label text="Helloworld heavy" FontFamily="{StaticResource HeavyFont}" />
    <Label text="Helloworld normal" FontFamily="{StaticResource NormalFont}" />
</StackLayout>