Hi,
Well, you can restrict the custom address fields to only shipping and only billing via the Display>Custom fields menu.
So if you do that, then you can just do like this:
{address_company}
{address_firstname} {address_lastname}
{address_street}
{address_post_code} {address_city}
{tax_number}{address_telephone}
That way, since either of the field will be empty on the last line, you'll only get the remaining field displayed.
Now if you want to keep the fields in each type of address but only change the display, it will require some PHP coding.
{address_company}
{address_firstname} {address_lastname}
{address_street}
{address_post_code} {address_city}
<?php if($this->address->address_type=='billing')){ echo '{tax_number}'; }else{ echo '{address_telephone}'; } ?>