I have a 10 gallon batch currently fermenting in a 15 gallon sanke on day 3. I used WL BG Golden Ale yeast. I want to use Brett in half of the 10 gallons.
Options are
1 - ferment out completely, keg half and rack other half into carboy and/or 5 gallon sanke and add Brett
2 - move half into carboy/5 gallon sanke before primary fermentation is done and add Brett
3 - ferment out completely, keg half, add brett and bottle condition with brett
My way of thinking about the timing is the Brett impact and your desired character. Earlier in ferment it seems the less overt Brett character you get. (Co-pitched with Sacc primary ferments are very clean). Having said that - Orval is pitched with Brett at packaging and it develops a lovely Brett character, but over the period of months+years.
I found that letting the beer completely ferment out then transfer onto Brett and estch for proper pelicle formation..
I would ferment it out dry and then add brett. You can pitch brett into a keg or fermenter with the five gallons and let it do its thing. You can also pitch in a bottling bucket, rack and bottle. Be mindful that brett will produce more CO2 so you run the risk of excessive carbonation that way. Orval does that but those bottles are thick and they’ve been at it long enough to know the amount of priming sugar and age to get the level of carbonation they want. From a fermentation vessel, you can prime pretty much as normal during your bottling process. Depending on how long you let it bulk age in secondary, you may want to add additional priming sugar to make up for the CO2 that will release from solution over time. When I bottle sour or brett beers, I typically age for a year or longer and prime bottles at 1.5-2x the normal amount.