In order to run multiple devices at the same time, you just need to put adb commands in shell script and made earlier ones run in background. In the adb command, specify device by their serial numbers. For example, create test.sh, in which you have
adb -s shell am instrument -w com.mytest.test/android.test.InstrumentationTestRunner&
adb -s shell am instrument -w com.mytest.test/android.test.InstrumentationTestRunner
The best part is that by combining this with auto build and deployment (hint: apache) , you can get fully automated test on multiple devices.
Video below shows a test suite of three very basic tests running on three devices (two physical and a simulator )at the same time.