This is a bit more robust (it won't fall over if you have no images open or can't find the corresponding image) I've made it so if you have the thumbnail image open it will find the mannequin image and visa versa. Save the script to your PS scripts directory. In photoshop record an action that uses the script. Stop recording! Then you can run the script as an action.

app.preferences.rulerUnits = Units.PIXELS;


if (documents.length == 0)
{
    alert ("No documents open, dude!")
}
else
{
    var dir1 = "Mannequin";
    var dir2 = "Thumbnails";

    // call the current document
    var tempDoc = app.activeDocument;

    // get the info out of the source doc
    var fileName = tempDoc.name;
    var filePath = tempDoc.path.toString();
    var fileNameAndPath = filePath + '/' + fileName;
    var tempDir = filePath.substring(filePath.lastIndexOf("/") +1, filePath.length);
    var goCode = false;
    var docName = fileName.substring(0,fileName.length -4);

    if (tempDir == dir1)
    {
        var otherImagePath = filePath.substring(0, filePath.lastIndexOf(dir1));
        var tempFile  = decodeURI(otherImagePath + "/" + dir2 + "/" + fileName);
        goCode = true;
    }

    if (tempDir == dir2)
    {
        var otherImagePath = filePath.substring(0, filePath.lastIndexOf(dir2));
        var tempFile  = decodeURI(otherImagePath + "/" + dir1 + "/" + fileName);
        goCode = true;
    }

    if (tempDir != dir1 && tempDir != dir2)
    {
        alert("Cannot find matching image.")
        goCode = false;
    }

    if (goCode)
    {
        //open the other image
        var fileRef = new File(tempFile)
        if (fileRef.exists)
        {
            app.open(fileRef);
        }
        else
        {
            alert("Matching file does not exist.")
        }
    }

}// end doc check

Hope this helps.