From what I understand, there is only enough space behind Main Street to store one parade. So on days when there is one Soundsational/A Christmas Fantasy and one (or two) PtN, Soundsational/A Christmas Fantasy will travel from Main Street to IaSW and then the first PtN will go from IaSW to Main Street. If there is a second PtN, it will go from Main Street to IaSW.
For days when there are two afternoon parades (e.g., Thanksgiving when A Christmas Fantasy is being presented twice) and two PtNs, I think the first afternoon parade will go from IaSW to MS and the second will go from MS to IaSW. Then the first PtN will go from IaSW to MS and the second will go from MS to IaSW. This way, there is only one parade behind MS at any given time.