Kudo to @enxaneta, the answer is:

<div>
    <svg width="64" height="64" fill="currentColor">
        <!-- 100% -->
        <use href="#bi-heart" width="64" height="64"></use>
        <!-- 50%, centered -->
        <use href="#bi-stag"  width="32" height="32" x="16" y="16"></use>
    </svg>
</div>