weight training 2x a day?

I don't do this, but I'm just curious...

Is it harmful/helpful/useless to weight train 2 times a day?  Say, at 7 am and 7 pm every other day?  I'm not talking an all out 40 minute training session, but say you had 20 minutes in the morning, and 20 minutes in the evening, would it be okay to train the same muscles twice a day?  I realize your muscles need time to recover in order to build, but assuming you are getting proper nutrition, would doing a morning/evening routine every other day work like one longer routine once a day?

Same muscles twice a day has always been a no, to me. When I have to split time up, I would dedicate say the moring 20 mins to my upper body and the 20 mins later in the evening to the lower body.

what she said^^^^^ i would focus on different muscles during different sessions.

It can be done, you can't do it with the same duration or volume, but it's definitely doable.

hmm..not sure why this double posted...

Thanks!  I really have no intention of doing this, I was just wondering if there was some accepted practice with this.  I know doing cardio 2x a day can be beneficial so it got me thinking

I'm out of shape and just started doing assisted chin ups to increase my upper body strength.  I pull about 80-90% as hard as I can and can do about 10 at a time doing that.  But since I know that's not a considerable workout I've been doing that like 5-6 times per day spread out every couple hours.  I think it's bad to do intense workouts on the same muscles twice a day, but if you're not working out really hard then I think it's alright.