I would like to know if it is ever an acceptable practice to mix or blend two diverse thinsets together, and under what circumstances one may resort to this practice (outside of desperation to finish a job in a shorter timeframe)?
Specifically, I'm interested in Laticrete 125 with Mapei Ultralite Mortar Pro, in any ratio, as this is what was done.
I commissioned my 1100 sq ft floor (on a post-tensioned concrete slab on the second floor of a condo) to be tiled with 18"x18" porcelain tiles using Laticrete 125. So far, all of the floor has been installed with 125, except it seems that in my ~70 sq ft master bathroom, the installer has diluted the Laticrete 125 with Mapei Ultralite Mortar Pro, in a ratio that appears to be 1/3 Laticrete to 2/3 Mapei, although I would say it was likely mixed a lot less scientifically than that. I believe this was done to save time, since he was unfamiliar with 125 and did not know how much coverage he would get up front, had not ordered enough 125 at the start of the job, and so he resorted to doing this without consulting his boss or me about it.
Also, it appears the trowel used in the bathroom has 1/4" notches, while the rest of the apartment was installed using a 1/2" notched trowel. My understanding is that 1/2" is recommended for use with these larger format tiles.
As an engineer, I realize that it is impossible to know for sure what any blend of products would do without proper testing, but I'm hoping there may either be someone who has experienced this before, or may know why it is either acceptable or never a good idea to do something like this, as I am not an expert, or even experienced in this area.