I'm not aware of those adpaters, can you share a pic or a link?

Maybe some, especially if the BB is low-ish already with 700c.
If you can fit 42 mm tires (not likely), then the height will be close to the same as with 700 x 23c.
622/2 = 311 + 23 = 334
584/2 = 292 + 42 = 334 (though that neglects increased sag from lower tire pressure, so the 650 is still a bit lower)

42 mm tires are heavenly on a tandem, but you generally need chainstays that are both indented and longer than modern racing bikes. Unless they're curved like a MTB. The only 700c racers that can be reliably converted to 650 x 42b are older, like early '70s when stays were longer and indents were still common. I have currently in the shop two PX-10s that fit 42 mm, a '69 and a '72, and a '74 Moto Grand Record fit's 'em too. The '71 Raleigh Super Course hanging next to them would not fit over about 35-38 though, despite the long stays, because they weren't indented. (I fixed that, by indenting them.) So you need both long and indented, unless curved, a feature no road racer has.

So it's likely you will top out at 32-35 mm, maybe 38 if you have indents in the chainstays. So you may lower the bike by around a centimeter. Whether that's a problem for you depends on crank length, Q-factor, and whether you like to pedal though corners. Q-factor doesn't have a huge effect but it's worthwhile trying to keep the cranks as narrow as possible, because excess width contributes to pedal-strike likelihood when you lean it over. There's also increased risk of "high-centering" due to the long wheelbase — you may hit from riding up or down a curb, or even some speed-bumps. So level the cranks when crossing them.