A connection cannot be made. Ensure that the server is running
After spending lot of time, I found the solution!

As I mentioned, I had to log into Win10 with User10 and at the same time log into Win7, using User7.

Looks like connection was successful when I log into both PC with the same username. 

So, I ended up to run SSMS as different user (in each PC) to be able connect to the other one. 

On Win10 PC, I created a shortcut to run SSMA as User7, and this way I was able to connect to WIn7 PC and also created the shortcut in Win7 PC to run the SSMS as User10 to be able to connect to Win10 PC.

This link (FIX Connection) also might be helpful, it did not help me though.

This is the link to create shortcut to run as different user