Yes you can do that. You can Get a Common Legend.
Please see the screenshot and attached. In the Screenshot I filtered Top 10 States, but have legends for all 50. For this you need to create another Sheet as in Sheet 1 in attached and use its legend in place of Legend of Sheet 2. When you Float Sheet 1 to Dashboard, send it back to Sheet 2 by Floating and using Send to Back Option from Dropdown for Sheet 1.
Hope It Helps!!!
All Legends.twbx 1.2 MB
That works ! Not exactly how I expected it to be done but that's fine.
Thank you very much