You can easily make 20 vol from 30 by mixing it with 33.3% distilled water, but I wouldn't try making 20 vols from 10 unless you have a laboratory and the correct equipment to distill accurately. One...
No, 10 is half as weak as 20 volume.. if you mix them equal parts you'll get a weak 15 volume...if you need 30 volume and only have 10,20 mix your color with just less than 2 parts 20 as in 1 oz...