I know a couple people who have ended up with spawning sixlines in tanks as small as 40 gallons. Both of them started with very tiny juveniles that they saw getting along in a dealer's tank. Same genus and similar behavior (aggression wise), so you may have luck trying that. Unfortunately I don't foresee walking into a fish shop and seeing two mystery wrasses in the same tank!
You could try getting two very small juvies though, and see what happens. :? Best of luck!
I've heard various suggestions about how to sex them (e.g. pattern on the face, existance of the five bars, etc.), but none of them seem valid. I don't think this species is really sexually dimorphic.
I had 2 in a tank for well over a year and they got along great. I also know of a few other people with more than one doing very well. I often wonder about the gender question too, but I am yet to read anything solid that leads anywhere.
I'll get my two at the end of this week. They were probably caught together. They are together in the importer's tank now, I've observed them for a few minutes yesterday, I didn't observe any quarreling, so I assume that it'll be a mated pair. The importer didn't know this either, he even called the exporter
I'll take pics before I release them in the tank. Don't expect these before thursday or friday.