It is not possible for two instances to share the same file system.
You can attach the same disk to two different VM's but not at the same time. In order to attach the disk (containing mentioned filesystem) to another machine first you need to detach it from the current one.
You'r impression of a "shared" filesystem must be false and you got confused (logged in twice to the same machine).
But - assuming there's some but (which may never be ruled out completely) you can check which disk is attached to which VM by running these commands in the cloud shell:
gcloud config set project my-project-name-1
gcloud compute instances describe instance1 | grep disk
Your output should be something similar:
wb@cloudshell:~ (wb)$ gcloud compute instances describe instaince111 | grep disk
Did you mean zone [europe-west4-b] for instance: [log1] (Y/n)? n
No zone specified. Using zone [europe-west3-c] for instance: [instance111].
Then select another project and run the describe command again.
----- UPDATE ------
Cloud shell is your personam VM that doesn't belong to any of the projects and is not in your default VPC. It's just created and preconfigured with
gcloud for your convinience. If you want to log in to specific VM it's best to use
SSH button next to the VM specs in the cloud console vm list or
[gcloud compute ssh] command.
If you just select another project in your cloud shell VM you're still logged in to this VM and it only tells
gcloud utility that any commands issued after
gcloud config set project my-proj-name will affect the project