Yes, at higher levels specially
This option would, simply, make Ranger (and every other class with access to this option) stronger by allowing them to take extra features at no cost.
I've had this problem when trying to combine different monk archetypes, and also on sorcerer. Your option would be ok at lower levels for ranger, but at lvl 20 it would mean extra features at no cost. What's more, some specific classes could get two features with excessively good synergy and be a bit broken, or simply be too weird to combine (two sorcery origins would be difficult to explain, and since they grant strong features at level one a sorcerer with 5 origins at lvl 5 would be really overpowered). Opening the door to this would make some classes vastly outclass others due to the sheer amount of features they get and their synergy. And restricting it to only ranger would be seen, most likely, as favouritism.
In the ranger example you could get at level 18 "any ranger archetype 15 / another ranger archetype 3" and get an additional archetype feature while losing nothing, since the last archetype feature you get is at level 15 and the first at level 3. Giving features for free isn't balanced (if done for all classes most likely it still wouldn't be balanced due to archetye features timing, but that's a different question).
So, instead, I'd recommend creating your own combined archetype, with features from both. The easiest way would be to choose one feature from one of both archetypes at each level where you get a feature, but that's a bit boring and might be exploitable. Making a brand new custom-tailored archetype is the best way to prevent weird unforseen combos. That way you can get what you want while following the rules and preventing exploitation. Work with the player to make the archetype he wants, make sure it's balanced and not only will you have his gratitudo for taking the time to tailor an archetype for him, you'll also have a little homebrew you can reuse on other campaigns.
Also, maybe your table doesn't care having a little (more) imbalance between classes, or won't get to a high level, or simply want the characters to be more powerful in general. In any of those cases your rule could work, though I'd still consider giving specific bonuses instead to avoid excessive min/maxing, and maybe simply letting players get the full benefits from two different archetypes would be simpler and preserve a bit better the balance between classes (albeit being an even more overpowered houserule).