Your local hardware store sells BRASS 90 degree and T fittings in the plumbing department that is see REGULARLY used just like in the picture above. Don't know their actual pressure rating but they work without incident. They are the 1/8 NPT thread fitting that is the same as paintball hoses, asa, and remote hoses used in paintball.
Stay away from the galvanized plumbing fittings as I know they are rated 300 psi only.
You shouldn't have any trouble finding a paintball 90 degree fitting. Stay away from Macroline and their fittings as they are chronic leakers in my opinion. Used the braided stainless steel paintball hoses and use with BLUE Loctite. You will never have a leak.
In the picture above, he used TWO remote hose fittings with the screw in type of on/off valve.
Now you CAN have tank problems that you don't want to have that will take you out of play. You might want to consider being able to ISOLATE each tank and not play with it or even have it taken off for repairs and not ruin your weekend..
You could blow a burst disk out of one tank during a refil. A reg could fail and start leaking air. You would want to remove the tanks quickly and easily for air fills, o-ring replacment, burst disk replacement, without being out of the game.. The tank reg could just STOP working and you would be out of play if you don't plan ahead.
I would use a bottomline ASA off a marker to go onto the tank. Lots of those cheap and available used in the drawers at your local paintball store left over from the drop forward upgrades. They also have convenient holes drilled in them to fix them on a mounting board if you want. However, if you unscrew ONE tank, the other tank will vent out through the OPEN ASA.
A ball valve would address that problem. Here is a link: 1000 psi rated $15.
Actually, you could just put one of these between each tank and the T fitting and isolate either tank and remove it if you wanted too. Cheaper than buying 2-3 remote hoses just for the fittings...
I would be inclined to go that way in terms of playability and cost to set it up.