Unfortunately the (open-url:) macro is implemented in a way that forces all URL's it opens to be within the same (unnamed) window/tab. You can use the Javascript window.open() function to manually open a URL in the same way that the macro does, by simply placing code like the following within your passage.
<script>window.open("http://google.com/", "name-of-window");</script>
... you will need to replace the "name-of-window" value in the above code with a unique name for each of the windows/tabs you want to have open at the same time.
WARNING: If the end-user is running Popup Blocking software (like that built into Chrome) then it may automatically stop those other windows/tabs from opening.