Hi Angel, do you mean do all of S&H in one day and then all of GS in one day? That's not the best for your body, as Cathe even explains part of this in the intro to S&H, the reason it's split up the way it is, is so your working opposite body parts. Like chest and back, you work your chest, and back together as they don't work off each other, which means you can go longer and make each one stronger faster. Where as if you worked chest and bicep/tricep you working muscles who first stablized the weights then your forcing them to work. You'll lose a lot of strength IE you'll have to go with lighter weights once you start working those muscle who stablized as they already had been worked some.
Like if you do a bench press, your working your chest, but your shoulders, and bicep,triceps, forearms, and wrists are stablizers for that weight your lifting up and down, while you work that chest. Most people who try to do this don't get a very good muscle build, mostly because you work those muscle hard as stablizers they can't do the actual exercises with the same heavy weights and good form, as they could if you seperated them like in both GS, SH, and PS do. Now you can always go for ligher weights and do this, but then your not getting the true benifit of the exercises or the what the workout was created for. You might want to put S&H in again and listen to the intro, as I think Cathe answers your question there as well.
Also the way these exercises are structured and done, makes it so your brain can send muscle fiber to just one or two body parts. If your diet is good you usually got enough store to help build two muscle groups and give them all the muscle fiber they are calling for. But more then two, then everyone has got to fight for that fiber, and no one gets built up like they should. Because you just don't have enough to go around so the ones you work later may be the ones that get worked and don't get any extra muscle fiber for their effort. Just think of it this way, drop a large box of pizza in front of 30 hungry kids, and see who gets a piece. This is kind of what your doing when you work more then two muscle groups to *exhaustion* you only have a limited amount of resources to build muscle fiber, so they either have to share, or it's first come first serve.
Hope that helps,
Kit