Angular template-driven form custom validation

I need to write the custom validator for the Angular template-driven form where one any non-empty input would be sufficient to make the form valid.

Consider the following Angular form with the two input fields and the submit button:

<form name="myform" (ngSubmit)="userForm.form.valid && saveData()" #userForm="ngForm" novalidate>

    <input type="text"
        [(ngModel)]="phoneNumber"
        name="phoneNumber"
        #phoneNumber="ngModel"
        pattern="\+[1-8]{1-3}\([0-9]{3}\)[0-9]{7}" />

    <input type="text"
        [(ngModel)]="privateEMail"
        name="privateEMail"
        #privateEMail="ngModel"
        pattern="^\w+([\.-]?\w+)*@\w+([\.-]?\w+)*(\.\w{2,6})+$" />

    <button type="submit" [disabled]="!userForm.form.valid">Submit</button>

</form>

The requirement - to make the form valid (additionally to the inputs pattern validation) if one any of both inputs should be not empty. To write the custom validator for the input controls has no sense - it would be limited with the scope of its own values only. To subscribe to the form event statusChanges brings also noting - this info is read only. When to write the control custom validator (like for the input control) and apply it to the form then it would not work - it seems to would not be called by the form.

What is the proper way to write the custom validator for the Angular template-driven form?