Technically, you can execute multiple processes in parallel but Anaplan will only execute the individual actions in serial. They will run in the order that they were run.
What I haven't tried but might work is if the SOURCE of the data is in two different workspaces. Each API call is defined by the workspace and model so it might be worth a try. But within the same workspace it is serialized.
Here's another article confirming this.
You may be interested in CloudWorks, where you can configure multiple cross-model imports and schedule them.