Yes, I have several App Engine instances with my Parse-on-express server. It can go from 3 instances to 20. I use the database options to ensure each server has its share of MongoDB connections, but not more.
The batches are started using App Engine Cron calls, which makes a single instance run each of them. Because of this, I had to buff each instance a bit. RAM isn’t that expensive and change performance a lot.
In the future, I plan on adding Google Cloud Tasks, which looks really simple, to distribute the batch load between instances with great control.
To be honest, I have split the instances serving the front site and the instances serving the back office/batch tasks.
Also note that each express server also serves the React static files.