Q:

Can I share a volume between two ec2 instances? (Windows and *nix)

I'm not really proficient with Linux administration, just random things.
I come up with the situation when i need to have potentially one volume which is accessible from one win box and one *nix box both hosted on amazon ec2.
More specifically I need to setup *nix box with about 1k accounts for SFTP and make files users upload to their home folders accessible the most easy way to windows box.
So I suppose mounting *nix volume in windows box is the method I need.
Can you point me to kind of walk-through on this kind of setup or explain me what actions I need to do.
After some Googling I got sense that i need to setup samba on *nix box, correct me if I'm wrong

A:

I think the best solution would be to use Samba (SMB) on the Linux machine and share that folder with the Windows machine.
I don't know which Linux distribution you use so I can't point you to the right Samba/SMB tutorial but any tutorial showing how to share files via SMB with a Windows machine should work.
The only additional tricks needed in EC2 are:

You need to open up the following ports on the security group of the Linux machine () and allow the Windows security group access to those ports.
When accessing the Linux machine from Windows you'll need to use the public DNS name of the Linux machine or its internal IP address (you can get both from the AWS Console).

