engths*:

>>> from itertools import combinations, repeat
>>> iterables = [combinations(range(4), 2), ['a', 'b', 'c']]
>>> lengths = [4 * (4 - 1) // 2, 3]
>>> list(interleave_evenly(iterables, lengths=lengths))
[(0, 1), (0, 2), 'a', (0, 3), (1, 2), 'b', (1, 3), (2, 3), 'c']

Based on Bresenham's algorithm.
Nz^Iterable lengths could not be determined automatically. Specify them with the lengths keyword.z,Mismatching number of iterables and lengths.c