I have a 20 year old house with manufactured floor joists that are 24" on center. Above this is ceramic tile which has cracked badly. I am told that the spacing of the joists is too wide to use tile. I know there are also materials that could have been used under this to make the tile work (used it in a new bathroom) but I am looking for a way to strengthen the floor (there is some spring in it) prior to putting down a wood floor. I have heard of sistering the joists but don't know how that would work with the manufacturerd joists. I want to get the squeek out!
24"oc joists are in fact almost always going to be too weak for tile. The span of the joist is one issue but the spacing is the big problem. Although 3/4" plywood is ok for floors with vinyl or carpet, it has waaaaay too much deflection at that joist spacing for the application of tile.
Depending on the access and the span of the joists, sistering really won't solve the problem of the distance between the joists. I'd focus my efforts on adding a joist between each existing joist, reducing the on-center spacing to 12"oc. Provided your span isn't too long and you have 3/4" floor sheathing that'll make the installation of tile fairly realistic.