Hi Jeff,
I just semi-randomly found this:
which includes slightly more information:
Mod 710R: X-Y operation, right Vertical compartment switchable from Vertical to Horizontal operation. For use with standard amplifier plug-ins. Requires 5B10N Mod 711C.
Mod 711C: X-Y operation plus control of unblanking. One switch position allows operation of time base as unblanking generator. See 5100 Series, Mod 710R.
(manually transcribed by sleepy proofreader)
That same 1991 catalog page 78 mentions an app note that may or may not be relevant: "Simultaneous display of two independent X-Y signal pairs" / "5111/5A14N/5A15N/5A18N. Dual X-Y techniques, engine analysis." / order no. 52-AX-4114
More wordy words over here:
/g/TekScopes/topic/87584525
