Perhaps
yaxis_secondary
ytics_secondary
Or shorter, but not so "correct":
yaxis2
ytics2.
I'm in favour of the first one (typing a few more keystrokes
resulting in greater readability of code). Also maxima (at least the
new stuff) favours spelled out names, which is good imo.
Maybe the addition to userpreamble can be automatically set when
yaxis_secondary = true?
Ziga