Yes you can, and No you can't.

A great deal depends on the actual dimensions of the joists (with 2" x 10" being the minimum and 2" x 12" used in earthquake prone states.), the centering distance between joists (16" centers the largest) and if the Plywood subfloor is tongue and grove. In addition, the plywood's (or OSB board's) must be glued to the floor joists, as well as having their tongue and grove sections glued together. The fasteners holding the boards to the joists should be coarse thread screws at least 1 3/4" long. The final test is "deflection". With a 300 lb weight at the center point of a 30' span, the floor must not deflect more than 1 inch. Single sheets of plywood (or any kind of wood) have been responsible for tiles cracking soon after installation on/at/near the seams.

In shorter spans, the measurements are at a lesser ratio. A water test can help as well. Place a near full glass of water at (or near) the center point of the tile area and step a foot or so away and jump up and down (in a circle) around the cup. If water sloshes out of the cup, you should reconsider your options by adding another layer of plywood or OSB. When doing so, it behooves the installer to glue and screw (or nail with 1 1/2" galvanized roofing nails) every 6" in both directions and every 4" is better if you are "paranoid" like me.

Of course, there must be adequate expansion room around the perimeter (1/4" or more) and fixed objects, and an 1/8" gap between boards that are left open (and protected from hard fillers) or filled with a flexible caulk or TAVY "007" Glue if you are using my ceramic tile underlayment, since our industry frowns on tiling directly over/to any kind of wood substrate/s. Armen Tavy