Brett tends to produce more unique flavors and characteristic aromas when stressed. This is why beers pitched with brett in secondary tend to get funkier than beers with brett used as the primary yeast strain. I think you could get away with a 1 liter starter with one pack of brett. Let that go till completion for about 7 days. Before pitching some of that brett into your saison in secondary, sanitize a pint mason jar and lid very well. Swirl up your brett starter to resuspend the yeast, then carefully pour half of your starter (500 mL) into that sanitized mason jar.
This mason jar with your brett will keep for a while. You may need to feed it every now and then until you are ready to pitch it into your Orval inspiration. Voila, you now have 2 healthy pitches from 1 packet. Ask me where I got that idea from (the BN of course!).
