def shuffle_seqlets(s1, s2):
    """Shuffle the seqlets among two seqlet groups

    Args:
      s1, s2: StackedSeqletContrib

    Returns:
      tuple of 2 StackedSeqletContrib with the same length as before
      but with seqlets randomly chosen from one of the two groups
    """
    return s1.append(s2).shuffle().split(len(s1))