Is it okay to use one power supply for the motherboard and a different
power supply for the hard drive on one PC?
Thanks.
Sure but it's a lot of trouble to go through unless you have a really
compelling reason to do it. A few people have done that for some
fancy case mods where the extra fans, neon tubes, video displays,
animatronic fuzzy dice, etc drew too much current for even a single
high end PSU. What you're proposing probably won't need you to go to
such extremes.
If it's just that you've run out of connectors, go to your local
computer shop and buy either a Molex Y-cable, or a Molex Octopus
cable. Locally, it's something like $6 for the Y-cable, and $25 for
the Octopus cable (splits off a single Molex connector into 6 to 8
connectors). Your local pricing will vary.
If it's that you think your PSU isn't powerful enough for what you've
got and that one hard drive will push it over the edge, you'd be much
better served by replacing your existing PSU with a beefier one. Saves
having to figure out how to mount the second one, as most cases don't
have a cage for more than one.