Python combine two integers to a third
You can use more built-in characteristics of the strings.  This code grabs each pair of digits and builds a list, which it then joins with no separator.  Then it adds the remainder from each string.  The shorter string's remainder is the empty string.

def int_merge(a, b):
    str_a = str(a)
    str_b = str(b)
    min_len = min(len(str_a), len(str_b))

    # Grab pairs of numbers while the lengths of both strings allow.
    # After that, add the remainder -- the shorter string will have none.
    result = ''.join([str_a[i]+str_b[i] for i in range(min_len)]) + \
             str_a[min_len:] + str_b[min_len:]
    return result

test = [
    (1548, 369),
    (1356, 24),
    (13, 2456)
]
for case in test:
    print int_merge(case[0], case[1])