You can definitely run more than one campaing at a time. For the best results and the most accurate data tracking, we suggest you only run one campaign at a time on a single element on your site.
This essentially means that, if you want to test different button designs for your "Add to Cart" button, then add all the buttons to a single campaign pointed at your add to cart button. You can still run another parallel campaign for a different button on the page, but it is ideal to keep a 1:1 ratio between campaigns and the page elements you are replacing.
To run two campaigns simulataneously, you can create two separate campaigns with different settings for the element on the page to replace. On shopify, this would mean selecting "Custom Element" for "Shopify Theme" and placing the CSS selector of the element in the settings. For other platforms, you would just need to specificy the CSS selector in the "CSS Selector of button to replace" setting in the same fashion.