Ask: Is it possible to combine two HD trees of key pairs to make a third tree?

1. PersonA locally generates an HD tree of key pairs (like a crypto HD wallet)

2. PersonB does the same resulting in two independent key trees

3. Together they create new key pairs for a third independent tree by combining their key pairs, for example:

(a) PersonA generates a key pair at specific path from their tree

(b) PersonB generates a key pair at same path from their tree

(c) PersonA and personB combine the public keys they generated _using some cool process_ to create a third public key

(d) PersonA and personB combine the private keys they generated _using some cool process_ to create the private key for that new public key

Anything like this possible?