For a 2nd floor studio loft we used 3 x 6 dbl T&G, solid decking (c.2 5/8" x 5 1/2" face). It was pre-drilled through on the horiz for the 8" Pole Barn Spikes,Ring Shank.
4" PBS, RS for edge nailing to the joists/beams. Nailing was straight forward, a good workout. Stay loose. Get some fat nail sets - those L shaped Japanese ones are good or a punch w a 1/4" diam tip.
Because of the 8 ft OC beams under the floor we needed to use the solid 3 x 6 w 2 tongues & grooves on each edge. 8" spikes connect and stiffen board to board to board. This met load and deflection code for 2nd fl residential (for 8' span of the decking).

I've also used 3 x 6 single T&G, laminated (3 stacked 1"x 6" pcs glued, w middle pc pushed sideways c. 3/4" to create the T&G.) Using Doug Fir, this decking from Potlach spanned joists/beams 5' OC for 40#/sf load and 1/360 deflection for code for 2FL Res bedrooms. This material is end-matched so joints don't have to land on joists. Significantly less expensive than the solid. Shop around.

I got these products in DFir from Billerica MA, straight and dry. Be prepared w some short cut-off pcs using the groove edge and your mallet or lump hammer to persuade and snug up an occasional piece before edge nailing.

I've also bought 2 x 6 solid T&G in Spruce, Pine & D Fir locally in NY and CT. On all these remember the appearance face is down, shows to the room below as finished ceiling. No dirty hands or shoes allowed on the material while it's stacked inside or well covered outside. If a pc needs some touch-up cleaning or sanding do it on horses before installing it. Have some cedar shingle tips on hand for cutting shims, as needed.

"I would rather supply and lay 2 layers of 2x decking -- is that an option?"
If you wanna do that you should install one layer perpendicular to the joists/rafters and one layer diag. Nail the 2nd layer thru the first into the framing members. You'll have a lot more waste this way, as the 2x decking will have to be cut to land on framing, it's not end-matched.