Thank you for your question.
It is true that the exception in section 22.214.171.124 of the Guidelines and Standards for Tactile Graphics says, "If one part of a tactile graphic requires more than one page for a key, the graphic would be placed on the left-hand page and the key on the following right-hand pages." However, in this case, I think because it will fit, it is preferable to have the key and the graphic on the same spread (facing left- and right-hand pages). The guideline still definitely applies if there are two key pages in addition to the graphic page, but nothing in the guidelines prohibits placing part of the key on the page with the graphic.
Also, depending on the length of the explanations for the keys (words versus longer phrases), it might be possible to put the key in 2 columns (G&S 126.96.36.199) and keep the key to one page.