I have purchased several sets of Mercury and ax-lightness (4 sets) and the two are very different. The ax-lightness are light, but they mold the spoke holes, rather than drill them.
Mercury is an outstanding value, but I didn't think for a tandem, the same as an ENVE. I was wrong.
I heard ENVE was a premier brand and someone said they molded holes too. Not in mine. They are drilled.
I have ENVE 24hole rear, 20 hole front, internal nipples - tubular.
I had them built tightly by a guy who has been building 30 years. I built 100s myself. My guess is he builds to a feel that is about 120kgf. I like higher as the ax are 140kgf.
I broke a rear spoke, at the rim/nipple. That kinda surprised me. 1st time in 20 some years. The reason is the sharp hole edges knick the spoke. For the money - ax-lightness are worth what they charge.