question on kegging with single regulator/multiple kegs

It does take time and pressure to carbonate beer. You mentioned you threw 30lbs on the oatmeal stout. How long was it on? I think most people around here will carbonate at serving pressure for a week give or take. You can carb faster by using 30PSI as you noted. Even though the timetables would be in days as far as I know.

 

There is no problem keeping some residual pressure on the Stout until you have your secondaries up and running. Yes, the headspace pressure will decrease as the beer absorbs the gas. I would top it off daily for the first couple of days, then set my target psi and leave it along.

If you're trying to fast carb the Stout there are a bunch of variables that determine the time (size of headspace, beer temp, residual CO2, how often you pour, etc). It's just about impossible to predict when fast-carbing, that's why you'll see a lot of advice to set and forget. In the week it takes to slow carb your Stout it's practically guaranteed to be improving. Pretty sure 12 psi on a Saison is under carbing (that's if the temp is really 43).

But I would do two things first:

1. Determine the beer temp. This will require a thermometer that has been calibrated. If you're a homebrewer you probably already have this.
2. Learn how to read a keg carbonation chart (many available online). You really want to know how many volumes of CO2 for each style of beer. Once you have these two figure (temp and carb level) you compute the psi to achieve this.

Usually a Stout and a Saison will have a big difference in pressure settings. With beers like these you really want to be able to control the pressure of each independently. Ignore everyone who says to set XX psi. Unless they have measured your beer temp and know the target carb level it's a shot in the dark. After you have temp, carb level, and psi, the last step is to cut your beer lines to balance the system. Caveat: nothing happens fast here, temps and pressure move slowly (think days). Expect a week or so to get it all dialed in. But kegging is wonderful, well worth the effort required to get it going. Good luck.

 

if you are trying to quickly force carb a keg with 30 psi, yes you will need to apply high pressure more than once. 30 psi once wont likely get you there but it is a start.

beware that you can easily over carb the beer with this method, so test it occasionally, and don't go crazy. maybe 30 psi 3 times over the course of a day or two. then drop it to serving pressure to even up the proper carbonation. you don't want to have your beer carbed to 9 vols.

Cheers.