Unfortunately this is another example at how misadapted and inadequate the test management tools and API's are in TestComplete. Nothing to do about it but write your own tools to handle it. Ironically the management tools in TestComplete are geared towards anything BUT automated testing. They require you to babysit the process from start to finish.
To achieve what you want, you need to run each test unit separately, which in the GUI involves pressing Play on the first test, waiting for it to finish, then pressing play on the second test. You can write your own test controller that interfaces to TestComplete trough COM and runs the tests you want. Took me about 10 hours and 1000 lines of VB code. I don't see why something similar is not already included in TC.
