Renovating the bathroom and have 3/4" shiplap for subfloor.
Tried to glue and screw 1/4" plywood to "beef it up" a bit and it just bounced in between the joists(16" o.c.) so I pulled it up before the glue could dry as I didnt want to continue on with porcelain tile and have them cracking.
Should I skip trying to add another layer of subfloor and go with Schluter Ditra where the tiles will be laid and leave the shiplap under the bathtub?
I'm not sure what code is in your part of the world, but here it's 5/8th over 5/8th (or 1.25 inches) like Just Bill said. I'd suggest screwing down 5/8" plywood over shiplap as there is significantly more movement there than there would be in a plywood subfloor.
Don't just add another 1/4" sheet of plywood, two sheets of 1/4" inch aren't as stable as one sheet of 1/2".
Good luck and have fun, it should look great when you're done!