Validate an input dependant on other input

I am trying to validate a text input that needs to be required if another text input has a value.

So for example:

<input type="text"  id="block1" name="block_1">
<input type="text"  id="numbering1" name="numbering_1" >  /* Required if block1 has value */

So I have a required validation rule applied to ‘numbering1’.
I also have a Required Dynaimic Attribute applied to it will the following binding:

block1.value != ''

See also screenshot.
Thsi is not working. So how should I be validating ‘Numbering1’ to be Required if block1 is NOT empty?

Thanks in advance.

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