Stainless Steel screws from underneath?

T&G decking is fastened with the screws concealled in the groove.

Screwing from underneath is screwing up. Screwing up is physically challenging. You're working against gravity; you're in an uncomfortable position while working. You also need to have someone heavy stand on each board above you to hold it in position, or use clamps, repositioning for each fastener. The installation takes about 3 times the labor. (Unless you consider the person standing above is taking a break, not working) Then it takes 2 x the labor.

And the screws from below do not hold the boards in place as well. The decking will cup, warp, and lift from the joist. Do not use Deckmaster concealled fastening system, either. Total crap product.

Decking must be screwed down, not screwed up. Screwing up will "screw up" the project.

"Screwing up will "screw up" the project."

My thoughts exactly but I was hoping she had a better explanation of what her true meaning was.

Sorry we did not explain better. The porch already has a deck of 2X6 treated on top of the joists. I was planning on laying the redwood boards on top of and perpendicular to the 2x6. There is good access from below. If I drill the pilot hole through the 2x6 from above, then put the board in place all I have to do from below is a small pilot hole in the redwood and then screw the screw in.

This is a bad idea. You may overload the deck structure by adding another layer over existing. Talk to an engineer or at least the building department, first.

T&G is fastened through the groove. The tongue of the next board fits into the groove, and is held in place. Then it gets fastened through it's groove. Basically, T&G uses only half the fasteners, because the boards are fastened on one side only.

The fasteners are all concealled with T&G. Why try to screw from below?

I have strained back and shoulder muscles drilling overhead. It doesn't take much. And I'm quite strong. I know several trades people who've torn rotator cuffs, required surgery, some permanently disabled. The cause is repetitive stress injury, from drilling overhead.