I am taking up carpet & particle board and putting a new layer of 1/2" plywood underlayment over the plywood subfloor prior to installing 3/4" hardwood (nailed down). I know I should orient the underlayment opposite of the subfloor's direction and that I should glue the underlayment to the subfloor (believe me, I will never reverse this install as long as I am in this house, so I am not worried about gluing it down). That said, I have also read that I should not screw the underlayment into the joists, but this seems mostly to come into play when folks are talking about installing tile. I am planning on using 1-3/4" subfloor screws to attach the underlayment to the subfloor. Do I really need to be careful about not hitting the joists with the screws? I am planning on 6" spacing on the edges and 8" spacing in the field. Also, should there be a gap between the underlayment plywood butt joints? Thanks.
First, I'd counter your plans to orient the second layer perpendicular to the subfloor. Plywood and OSB have a strength axis that should run across the joists. Instead of turning the top layer, simply stagger its joints with respect to those of the subfloor. Start with a ripped half sheet and stagger the butt joint by two joists.
I've never heard the advice you mention about avoiding joists with underlayment fasteners. It's probably not critical to run screws into joists, but there are a couple reasons why you would.
Running through between the joists creates some possibility that you'll hit something underneath that shouldn't have holes in it. Using 1-3/4" screws it's not likely to be a problem, but it is possible.
You may find your screws stripping at the threads before the heads are completely sunk. They should be at or below flush, and that takes quite a bit of pull from the threads.
I wouldn't worry about gapping butt joints with plywood. It's stable enough that it shouldn't be a problem unless you happen to be installing very dry wood in a very humid environment. OSB is usually undersized to allow for swelling, but not plywood.
You are referring to the "decoupling principle" which allows the surface to move independently from the substrate (what's underneath). It's generally not necessary with wood, because wood can slowly change shape as the house ages. Tile is rigid and crystalline and will never change shape, therefore decoupling is essential. If both layers of sub-flooring and the hardwood flooring were all anchored into the joists, it would prevent the house from deforming. If the upper layers are separated from a settling house, the upper layers will not deform as quickly, but will not add to the structural integrity of the floor.
Simply, your floor can hold up your home, or your floor can outlast your home.