It does not have one.
Bitcoin Core uses hardened derivation, so there is no way to compute the addresses it will use externally.
Support for that will likely be added in upcoming versions, but likely won't be the default (there are security risks when using non-hardened derivation).
If you are comfortable with using the command line then libbitcoin-explorer is a great utility for converting your bitcoin HD keys. It even works completely offline.
Once you have bx installed, you can run the hd-to-public command like so:
$ bx hd-to-public ...
This will be mostly possible in the upcoming Bitcoin Core 0.18 release.
First you need to understand that the sequence of such paired multisig addresses (with public keys generated from 2 xpubs in lockstep) can be described in the new descriptor language.
The syntax is sh(multi(2,XPUB/*,XPUB/*)). The sh indicates P2SH embedding, multi refers to multisig, ...
A multi-sig address is a hash of a script that combines multiple separate keys. In other words the purpose of it is to require multiple signatures from multiple keys that are stored (and usually generated) independently. For that reason it can not be generated from a single extended public key (xpub).
What you can do, is to have multiple extended keys, and ...