My Dual 1229 had that same dual-spindle option. I only used the stacking feature when spinning for frat parties (with other guys' records, lol).
Idler drive turntables
One sees all sorts of data re Garrard 301/401, Thorens TD 124, and Lenco turntables of the 60's and 70's. These TT's if restored bring lots of $ on the used market! What about the Benjamin Miracords of the same era? My first "serious" hifi system in 1970 consisted of a Miracord 50H with a Shure M91E cartridge, a Fisher 600 receiver and KLH 4 speakers. Sometimes, even now, and thousands of dollars more, I yearn for the sound of that system! Just how does the Miracord stand up to those icons??