Better algorithm to riffle shuffle (or interleave) multiple lists of varying lengths

I like to watch my favorite TV shows on the go. I have all episodes of each show I'm following in my playlist. Not all shows consist of the same number of episodes. Unlike some who prefer marathons, I like to interleave episodes of one show with those of another.

For example, if I have a show called ABC with 2 episodes, and a show called XYZ with 4 episodes, I would like my playlist to look like:

XYZe1.mp4
ABCe1.mp4
XYZe2.mp4
XYZe3.mp4
ABCe2.mp4
XYZe4.mp4

One way to generate this interleaved playlist is to represent each show as a list of episodes and perform a riffle shuffle on all shows. One could write a function that would compute, for each episode, its position on a unit-time interval (between 0.0 and 1.0 exclusive, 0.0 being beginning of season, 1.0 being end of season), then sort all episodes according to their position.

I wrote the following simple function in Python 2.7 to perform an in-shuffle:

def riffle_shuffle(piles_list):
    scored_pile = ((((item_position + 0.5) / len(pile), len(piles_list) - pile_position), item) for pile_position, pile in enumerate(piles_list) for item_position, item in enumerate(pile))
    shuffled_pile = [item for score, item in sorted(scored_pile)]
    return shuffled_pile

To get the playlist for the above example, I simply need to call:

riffle_shuffle([['ABCe1.mp4', 'ABCe2.mp4'], ['XYZe1.mp4', 'XYZe2.mp4', 'XYZe3.mp4', 'XYZe4.mp4']])

This works fairly well most of the time. However, there are cases where results are non-optimal--two adjacent entries in the playlist are episodes from the same show. For example:

>>> riffle_shuffle([['ABCe1', 'ABCe2'], ['LMNe1', 'LMNe2', 'LMNe3'], ['XYZe1', 'XYZe2', 'XYZe3', 'XYZe4', 'XYZe5']])
['XYZe1', 'LMNe1', 'ABCe1', 'XYZe2', 'XYZe3', 'LMNe2', 'XYZe4', 'ABCe2', 'LMNe3', 'XYZe5']

Notice that there are two episodes of 'XYZ' tha