If your MailChimp URL has a query string at the end (a ? followed by one or more &name=value parameters -- which I think MailChimp URLs do have), you would not be able to combine that with other URLs in the same field. However, you can enable "Send Transaction Data to a URL" in the settings of each individual product that should also send order details to another URL when that product is purchased. When you checkmark that setting for a product, click Next until you find the "Payment Variable Information URL" field where you can paste that URL; repeat this for each product that should send order data to another URL when it's purchased. Your Common Notification URL and the Payment Variable Information URL for each product purchased would each receive a separate transmission of the order data.
Another way of going about that would be to have a custom "middleman" script set up on your server, so your Common Notification URL would just submit to the URL of that script, which would then resubmit the data to whichever other URLs you may require.
As a side note, if you needed to post order data to multiple URLs which do not have query strings, you could use the passthru method documented here:
http://www.e-junkie.com/ej/tips.integration.built-in.htm#passthru
In that case, your Common Notification URL would look like this example:
http://www.e-junkie.com/ecom/oplug.php?postto[1]=http://www.example.com/script.ext&postto[2]=http://www.otherexample.com/otherscript.ext
