How to reference a DataTemplate inside a DataTemplate?

Question

I have this ResourceDictionary :

<DataTemplate DataType="{x:Type vm:MainViewModel}">
    <ListView>
        <ListView.ItemTemplate>
            <DataTemplate />    <----- Here I'd like to load an external DataTemplate from the same folder
        </ListView.ItemTemplate>
    </ListView>
</DataTemplate>

In the same folder I have another ResourceDictionary :

<DataTemplate DataType="{x:Type vm:EditRecordViewModel}">
    <StackPanel>
       <TextBlock Text="Test" />
    </StackPanel>
</DataTemplate>

Question

In my first ResourceDictionary how do I get the 2nd ResourceDictionary to display in the first, where I have the <DataTemplate /> displayed?

I have added resources in the App.xaml like so, but how do actually use them?:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Views/MainViewModel.xaml" />
            <ResourceDictionary Source="Views/EditRecordViewModel.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Answer 1

You can provide a key to a data template

<DataTemplate x:Key="test" DataType="{x:Type local:EditRecordViewModel}">...

and then reference it

<ListView ItemTemplate="{StaticResource test}">...

---

Answer 2

You have declared Data Templates without a key, which means that they will be applied to the corresponding types by default.
You don't need any links for this.
The only thing that matters is that the ListView gets a collection of items of type EditRecordViewModel.

Example.
TwoDataTemplte/ViewModels.cs:

namespace TwoDataTemplte.ViewModel
{
    public class EditRecordViewModel
    {
        public string Text { set; get; }
    }
    public class MainViewModel
    {

        public EditRecordViewModel[] EditRecords { get; } =
        {
            new EditRecordViewModel() {Text = "First"},
            new EditRecordViewModel() {Text = "Second"}
        };
    }
}

TwoDataTemplte\\MainDictionary.xaml:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:vm="clr-namespace:TwoDataTemplte.ViewModel">
    <DataTemplate DataType="{x:Type vm:MainViewModel}">
        <ListView ItemsSource="{Binding EditRecords}"/>
    </DataTemplate>
</ResourceDictionary>

TwoDataTemplte\\ItemDictionary.xaml:

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:vm="clr-namespace:TwoDataTemplte.ViewModel">
    <DataTemplate DataType="{x:Type vm:EditRecordViewModel}">
        <StackPanel>
            <TextBlock Text="{Binding Text}" />
        </StackPanel>
    </DataTemplate>
</ResourceDictionary>

App.xaml:

<Application x:Class="Febr20y.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:Febr20y"
             StartupUri="TwoDataTemplte/ExampleWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="TwoDataTemplte/MainDictionary.xaml" />
                <ResourceDictionary Source="TwoDataTemplte/ItemDictionary.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

TwoDataTemplte\\ExampleWindow.xaml:

<Window x:Class="TwoDataTemplte.ExampleWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:TwoDataTemplte"
        xmlns:vm="clr-namespace:TwoDataTemplte.ViewModel"
        mc:Ignorable="d"
        Title="ExampleWindow" Height="450" Width="800">
    <Grid>
        <ContentControl>
            <vm:MainViewModel/>
        </ContentControl>
    </Grid>
</Window>