It's up to you what body parts to work out. I think working out morning AND night daily is pushing it though. Your body does need rest...And allow at least 48 hours per muscle group before working it out again.
When a friend and I workout..If we workout our back, we'll workout our triceps as well. When we workout our chest, we'll do biceps also. Throw in abs and shoulders within either workout. That's just the grouping we like the best. A few of the machines we use though will workout the chest and biceps in one, so I'm assuming that's how we got into the groove of Chest/Biceps, Back/Tri's. There's really no set 'You should work this muscle group with this muscle group', it's up to what you feel like working out.