I'm on the dnx 1.0.0-rc1-final, xunit 2.1.0, xunit.runner.dnx 2.1.0-rc1-build204, and the option maxParallelThreadsto xunit.runner.jsonwork for me from the command line.
Do you have your tests shared by collections? According to the documents :
By default, each test class is a unique test collection.
Therefore, given some far-fetched โtestsโ like these, it should be easy to see if they work in parallel or not:

FROM
{
"diagnosticMessages": true,
"maxParallelThreads": 4
}

FROM
{
"diagnosticMessages": true,
"maxParallelThreads": 1
}
