Monolith, onion architecture, multiple database types, How to do "transaction" from 2 different service classes with different database types?

I want to ask about architecture

I use onion architecture. i have 2 big modules, User Module & Order Module

The User Module uses mysql The Order module uses mongodb

Both have service class.

When a user places an order, the Service class in the order module will call the service from the User module to update the User data.

How to do "transaction" from 2 different service classes with different database types?

Thank you