xsd 1.1 combining two assert statements using IF

I would like to combine the following two assert statements in to one, presumably using if? If ID = 8 zeroes then date of birth must be provided...

If have the two asserts working (as follows) but unable to combine them with an if statement...

      <xsd:assert test="(.//ID eq '000000000') "/>

then

      <xsd:assert test="(.//DateofBirth > xsd:date('1900-01-01')) "/>

Any help will be grately appreciated, thanks guys