http://mmgen54p3ouu7zymz5u3mwoncsyfpqeamfaaupeordl6ogujslk7fjad.onion/project/mmgen/mmgen-wallet/wiki/Subwallets
Being ordinary wallets, subwallets may be used to generate other subwallets in
turn, leading to hierarchies of arbitrary depth. However, this is inadvisable
in practice for two reasons: Firstly, it creates accounting complexity,
requiring the user to independently keep track of a derivation tree. More
importantly, however, it leads to the danger of Seed ID collisions between
subseeds at different levels of the hierarchy, as the software checks and avoids
ID collisions...