It won't happen. You're ignoring that BOTW isn't a very accessible game whilst Odyssey is. BOTW heavily panders to experienced gamers and casuals make up a lot of the gaming market, indisputably. Odyssey is more appealing to the eye on the shelf and Mario is far more iconic than Zelda. The IP power itself is prone to beat anything else on the platform.
We can already see that Odyssey has beaten BOTW on Switch in just under 3 months, where Odyssey is at 8m~ as of mid-Jan and BOTW is 6.5m~ in 10 months or so...combining Wii U sales won't do anything, the Wii U is selling under 1,000 consoles weekly and I doubt that that 1.3m~ ish figure for Wii U will increase much beyond; probably will stay like that or peak at 1.5m. So Switch + Wii U sales still don't beat Odyssey, and BOTW isn't charting higher either, which would be its chance at surpassing it.
BOTW has better legs than previous Zeldas due to all the buzz, but I guarantee that once games like Pokemon and Smash reach the platform, Mario will fare better on the charts when carrying that weight.
SMO is at 9.07mil years end and BotW is at over 8.5mil years end. They are close (2017).
Smo will win but botw is ironically doing well. Ita just smo is killing it. When you look at things in a better light even if SMO beats BoTW because 3D mario > Zelda... BotW can possibly outsell every 3D mario game to date (excluding Smo)
You people are heavily overstating BOTW. When more games release on the Switch, its legs will fall off. Heck, can the game even beat Mario Kart 8 Deluxe? The game isn't new; still charted better than both games in the most recent European weekly without holiday season legs. BOTW has been doing better than SMO 3 weeks in a row in Japan, but this is by a few hundreds of units, so I don't think it's surpassing SMO there either.