Mount FSx in EC2 of different aws account

There are a couple approaches that come to mind. If the subnet(s) (Windows and NetApp OnTap can be multi-AZ, while Lustre and ZFS are currently single AZ) of the FSx deployment are peered with the second account, or, if you share the subnets, you should be able to access the filesystem. You'll want to ensure you meet your security requirements between the accounts by having Security Group, Network ACLs, IAM Roles, Active Directory Authentication, etc. to ensure you don't leave vulnerabilities for access to things you don't wish to expose.