5/8 plywood is span rated for 20" subfloor.
if you put 2 layers of 5/8 perpendicular to each other, does it increase the span rating and if so, by how much?

 

2 layers would not change the fact that the first layer is still only rated for a maximum 20" span.

 

Won't it flex a bit less? I figured that's why tilers always ask for an extra layer of plywood for strength

 

Sure. But it doesn't mean its okay to use 5/8" on 24" centers if the APA rating is 20.

 

I'm confused. If it makes it stiffer then doesn't that make it stronger and therefore able to handle a slightly bigger span or is it just that it deflects less?

 

Keep in mind that with a rigid and brittle product like tile and grout joints it's not really about "strength". It's about stiffness. The floor certainly won't collapse but there may be too much flexing between joists which can cause the tile to crack, come loose and grout joints to crack & crumble.

The magic of plywood is that it's layers are completely glued together. That's where it gets it's strength and stiffness. Placing two sheets on top of each other doesn't double the strength as both sheets can slide/move in relation to each other. You do get an increase in strength but it's much less than if you had 1 1/4" plywood. Thoroughly gluing and screwing two sheets of 5/8" will provide the most stiffness and strength if that's what you gotta do.

If you are close on span I would go up in thickness. I'm a big fan of Advantech which is a premium OSB product. It is more dense and stiffer than plywood so for the same or less thickness you can have a stiffer floor. And as a bonus Advantech is much more water resistant than plywood which is a huge bonus for bathrooms and kitchens.

Placing another sheet perpendicular doesn't necessarily help because the layers within the plywood are already perpendicular to each other. In plywood sheets with an odd number of plies one orientation is stiffer than the other. In that case placing each layer perpendicular will help.