You should split it into 2 separate arguments:
parser.add_argument('-i', type=int, metavar='input', choices=range(1,65), help='set input.')
parser.add_argument('-o', type=int, metavar='output', choices=range(1,5), help='set output.')
Or, if you really want to use one argument, you can do the checking manually:
parser.add_argument('--io', nargs=2, type=int, metavar=('input', 'output'), help='set input (1 to 64) -> output (1 to 4).')
# ...
args = parser.parse_args(argv[1:])
input, output = args.io
if input < 1 or 64 < input:
raise ValueError("input:{!r} must be between 1 and 64.".format(input))
if output < 1 or 4 < output:
raise ValueError("output:{!r} must be between 1 and 4.".format(output))