If one of the tiles is fairly flat with minimum warping cupping you may meet the proper coverage requirement using a 3 8 square notched trowel.
Trowel size for mosaic wall tile.
Tile trowels come in many shapes and sizes generally starting out from a 3 16 v notch 3 16 to 1 4 v notch mosaics tiles up to 4 1 2 glazed wall tile.
Tile trowels come in many shapes and sizes generally starting out from a 3 16 v notch up to the 1 2 square notch.
Twelve inch square tiles for instance are best applied using a trowel with 1 2 inch by 1 2 inch notches.
The other main consideration is where the tiles will be set on the floor or on the walls and ceilings.
Mosaic tiles and wall tiles up to about 4 by 4 inches call for a v notch trowel with 3 16 by 1 4 inch notches while larger tiles require a trowel with a square or u shaped notch to obtain proper adhesive coverage.
If however the other tile has quite a bit of cupping you may need to use a larger 1 2 square notched tile trowel to get proper coverage.