I am planning to run feeder cable to a 240v subpanel for a future workshop about 90 ft from main. I may never need the 240 voltage but want to keep that base covered just in case. So, instead of a 120v 100 amp subpanel, I'll have a 240v subpanel with say... 60 amps each of the two 120v legs. Using red,blk,what #6 thhn w/#8 grnd from main to sub underground conduit. Is this correct? A shared neutral an issue? Voltage drop calculates at 2.1%. Thanks!

Pretty much all sub-feeds are 120/240v. There is no reason at all to run a 120v only feeder. In fact it raises several problems not faced with a 120/240v feeder, such as you will NEVER find a single-pole 60-100A breaker.