Normally, this would not be allowed. You will need to upgrade your license to increase end user number. In your case, though, I see you have those two 5 user licenses and we will make and exception and combine them for you.
In fact, I have already done so and now you have 1 license with 10 end users. As you have already activated a key, in order for the changes to take effect you will need to deactivate and activate your license. The process is explained in this video: http://tv.telerik.com//watch/team-productivity-tools/activation?PrivateKey=57588ee870305842
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