Unless you are using different agent pools, you can set up multiple TFS Build Agents on one build server (machine). If you are putting the agents in the same agent pool, your demand is not possible. There is a clear picture show in how are agent pools and queues organized?
From the above picture you can see unless they are in a different agent pool, there is no possibility to build two different requests at the same time.
Moreover, there is also a parallel build in the vNext build definition. However, this parallel build only has support for different platforms by different build variables.
