TFS 2010 Multiple Solutions & Build with Gated Check-in

@Gchaves found a solution for that problem, you can go to "Edit Build Definitions" and on "Workspace" tab if you configure your "Source Control Folder" to the inner folder, where you have SolutionA.

For instance, if you have this file system:

1. Projects\SolutionA\SolutionA.sln
2. Projects\SolutionB\SolutionB.sln

Now if you configure "Source Control Folder" and "Build Agent Folder" to Projects\SolutionA, the Gated check-in will only be triggered when you try to checkIn SolutionA, and it will only compile SolutionA.sln (for this to be true you must have to point just to SolutionA.sln on Process Tab ->1.Required->ProjectsToBuild)

And then you can have multiple solutions under same workspace, building, checking in and labeling independently:)