By being two archetypes at once. That's about it. It's perfectly acceptable so long as none of the modifications they make overlap.
As long as the Archetypes don't change the same feature you can stack as many non-conflicting archetypes as you like.
For example, Hexcrafter and Kensai both alter the baseline Magus' Spell Recall feature so they can't be combined. But since Hexcrafter and Bladebound don't have any overlapping feature changes, they're valid to be combined.
It's not up to date with all the latest splat books, but here's a handy thread post that lists valid archetype combos.
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Depending on the class, you can combine a crazy amount of archetypes. One such combination is Drunken Qinggong Master of Many Styles of the Iron Mountain. That's 4 archetypes tagged on the monk class! Which practically makes it no longer a monk, but whatever. So yeah, you can smoosh together as many archetypes as you want as long as two archetypes aren't trying to replace the same thing.
