Use a datastore on two OSes with esxi 6.7

-- I assume that by "datastore" you mean "virtual disk", because you can't attach a datastore to a VM; what you attach to VMs are the virtual disks that you create in the datastore. --

You actually can attach the same virtual disk to multiple VMs, there are config settings in VMware to allow that. But you will need a clustering solution in the guest systems to arbitrare disk access, otherwise the only thing you will achieve will be a corrupted file system; on Windows that would be Windows Server Failover Clustering, on Linux there are several options.

AFAIK it's not possible to build a cluster between different operating systems (nor would it make much sense); your best (and only) solution is to mount the disk on one of the VMs and use the network to share its contents with the other one.

Short answer: You don’t. Especially not across incompatible operating systems.

Long answer: Provided both machines know not to touch the same files or underlying sectors simultaneously, it could in theory work, but you don’t want to go there with a traditional multipurpose file system.

If both VMs need to access files on that vdisk, mount it on one of the machines and have that machine present a network share to the other. On the same host, and with vmxnet3 network cards in the VMs (requires VMware Tools or OpenVMTools), transfers will be as fast as the host can manage.