Is it possible to build a project for 2 platforms using a single
command?
It can be done in Visual Studio.
1) In VS IDE,
there is an option called Batch Build which can build different projects with different platforms,..etc at the same time.
Build-->Batch Build or right-click your solution-->Batch Build

You can choose any platforms for your project to build them.
2) If you want to use MSBuild command line,
You can create a proj called test.proj
file which assembles the association between the project and the platform.
Then build test.proj
in msbuild command line directly.
You can check my answer in a similar issue.
In my answer, BuildInParallel="true"
can build all projects in parallel, which can improve the performance of msbuild build.