It depends on the separation requirements for your two services. Withing one project, services share some App Engine resources. For example, Cloud Datastore, Memcache, and Task Queues are all shared resources between services in an App Engine project. If this is OK with you, deploying one service to the Standard Environment, the other to Flexible is possible. You need to specify env: flex in the app.yaml configuration file, if you need the service to be deployed to the Flexible Environment. Each service is reached by a different URL: https://[SERVICE_ID]-dot-[MY_PROJECT_ID].
appspot.com or http://[SERVICE_ID].[MY_CUSTOM_DOMAIN] . More about routing at "
How Requests are Routed". You may gather more insight by reading the "Microservices Architecture on Google App Engine" documentation
page.
Flexible Environment is characterized by slightly different requirements, when compared to the reference to the Standard Environment, so you may benefit by reading the "Migrating Services from the Standard Environment to the Flexible Environment"
page.