Why won't my Keyboard and Mouse work with 2 VM's Open

Hi everyone, I hope I am posting this in the right place if not really sorry.

I am trying to conduct a simulated Penetration Test for my course. I have one VM which is running Linux Kali 2.0 and another running Windows 7. As soon as the Windows 7 VM is running Kali doesn't let me use the keyboard and mouse anymore. As soon as I pause the windows 7 VM I can use them again within Kali. Why is this happening and how can I get them to work in both VM's. Both my Keyboard and mouse are plugged into USB 2.0 ports

Thanks in advanced and again really sorry if I asked this in the wrong place.