def fill_clusters(clusters, element_i, element_j):
    """
    Fill the list of clusters (sets) with two elements from the same cluster.

    >>> clusters = []
    >>> fill_clusters(clusters, 1, 10)
    [{1, 10}]
    >>> fill_clusters(clusters, 10, 100)
    [{1, 10, 100}]
    >>> fill_clusters(clusters, 20, 30)
    [{1, 10, 100}, {20, 30}]
    """
    if not clusters:
        clusters.append({element_i, element_j})
    else:
        found = False
        for cluster in clusters:
            if element_i in cluster:
                cluster.add(element_j)
                found = True
            elif element_j in cluster:
                cluster.add(element_i)
                found = True
        if not found:
            clusters.append({element_i, element_j})
    return clusters
