Unless they move a lot of their new Odyssey code into Horizons, I don't see how players running Odyssey will even be able to land on the same planets as players running Horizons. I mean, how would that work? Would Horizons players see a planet rendered using the current system, and Odyssey players see the new rendering features and atmospheric effects? It would be tricky to make the two renders compatible in terms of basic terrain features.
I see the possibilities for Frontier as follows:
1. Create a new set of planets that allow space legs, and use the new rendering tech, and only let Odyssey owners land there. Existing planets can still be accessed, but only via SRV (for everyone, including Odyssey players). So Horizons players can still land on all the currently accessible planets and see Odyssey players in ships and SRVs, but can't land on any of the new ones. Odyssey players get to land on the new planets as well, enjoy the new planetary rendering tech, and run around shooting at each other etc.
or
2. Add some of the Odyssey code to Horizons, so that Horizons owners can land on all the same planets Odyssey owners can land on, but can't run around with them. They'd see the Odyssey players running around shooting at each other, but could only sit in their ship or SRV as now, but they would get the benefit of the new rendering tech, with atmospheres etc.
In option 2, it may be possible to run Horizons in VR and get to enjoy the new planetary tech, but if this means Frontier have to do some work to get the VR to work with the Odyssey players running rings around the Horizons player's SRV, they may have gone with option 1 instead... Also Frontier may not want to give this amount of improvement to planetary renders to Horizons players who haven't actually paid for it.
Separating Horizons players from players of the base game is easier, as only Horizons players can land on planets anyway (which is probably why all engineers are on planetary bases).
If Odyssey is an optional DLC with all players still in the same basic universe, its going to get pretty complicated with players running three different clients with very different abilities. My guess is they'll upgrade the base game to Horizons, at least for the planetary side of things, and probably take up option 2 above, but there's always the very good chance that I'm wrong!