If two services share the same data (e.g. bank account data, shopping cart) and need to update the data transactionally, simplest approach is to keep both in the same database and use database transactions to enforce consistency. Any other solution is hard.
Mastering the 4 Balancing Acts in Microservices Architecture
Srinath Perera

Why not just have a single data source, with a single service serving/saving that data to however many services?

That way it keeps your microservices a bit lighter and more singularly focused.