How to apply one signature test to multiple positionals

There are a few ways to approach this but what I'd probably do – and a generally useful pattern – is to use a subset to create a slightly over-inclusive multi and then redispatch the case you shouldn't have included. For the example you provided, that might look a bit like:

subset RealOrMeasure where Real | Measure;
multi infix:<+> ( RealOrMeasure:D $left, RealOrMeasure:D $right )  {
    given $left, $right {
       when Real,    Real    { nextsame }
       when Real,    Measure { $right.clone.add-const($left)  }
       when Measure, Real    {  $left.clone.add-const($right) }
       when Measure, Measure { my ($result, $argument) = infix-prep $left, $right;
                               $result.add($argument)}}

}

(Note: I haven't tested this code with Measure; let me know if it doesn't work. But the general idea should be workable.)

You can add an Int - method to your class and use signature coercion.

class Foo { 
    method Int { 110 };
} 
multi t( Int() $x ) { $x + 1 };
multi t( Real $x ) { $x.Int + 11 };

say t( 0 );
say t( 0.1 );
say t( Foo.new );