TOTO Back to Back toilet installation

I'm replacing an old toilet with a TOTO Ultramax in a back to back installation (1961 plumbing). The installation notes state that TOTO recommends a double combination tee-wye/1/8 bend only, not a double sanitary tee/sanitary cross. How major of a concern is this?? I may have the older sanitary cross type fitting.

IT makes no difference what brand or type of toilet it is, a sanitary cross is not, and has NEVER been, the proper fitting for back to back ANYTHING. The proper fitting is a back to back fixture fitting, although for toilets a double combination Y-1/8 bend will work.

Hi,
I'm getting ready to install a Drake II to replace 1 of my Am Std 6 gal toilets (ca 1968). It will be "back-to-back??" with 1 of the old toilets. There is about 2 ft of drain on either side of the (cross-wye??) fitting to the stack and sewer drop to the basement.
Will I need to make any change to that fitting.

Picture of fitting...

Gerald,
That is not the best fitting for back to back toilets.
But in a pinch it would work. They can force air back, which pushes the water up in the bowl and when it drops, it may lose some water.
At least it does in my home.

My guess is yes, but wait for a pro. The newer toilets tend to have a pretty high velocity outlet and this can cause the discharge to cross over and do weird things with the toilet opposite it. That's why you need the proper fitting, it directs things down the drain, and won't allow it to cross over. Older toilets may have used more water, but it was a much slower exit than the new ones and thus didn't have (most of the time) enough energy to cross over.