code:

/*
* Gets a list of selected tokens and swaps their position if there are two selected
*/

let targetTokens = canvas.tokens.controlled;
function checkForTokens(){
    if (targetTokens.length === 2) {
        return swapTokens(targetTokens);
    } else {
        return ui.notifications.error('You must designate two tokens for switching.');
    }
};

function swapTokens(tokens) {
    tokens[0].update({'x':tokens[1].x, 'y':tokens[1].y});
    tokens[1].update({'x':tokens[0].x, 'y':tokens[0].y});
}

checkForTokens();