Double main with Megaproject support giving 1 would give us 3 total, though we'd probably need another to fully finish.
Remember this:
Written Code of Laws (Copper Age)
Bold, early innovators, the People have established an early written code of laws. While it will undoubtedly change over the centuries, this early establishment of what the rules definitely are with clear guidelines for punishment will serve as an important cornerstone for maintaining stability and clarity of order. Upgrades Enforce Authority to Enforce Law, doubles the strength of one Policy action per turn, and allows for the 'Megaproject support' Policy.
New social advancements have opened up new abilities for the king to direct lower level chiefs, opening up the Policy system
Policies give directive to the provincial chiefs and allow for greater flexibility in provincial actions, including combining the actions of multiple provinces into Main actions and spending resources even if not normally allowed (resource allocation will avoid reducing a stat to below 1). If there are not enough resources to continue a policy the chiefs will take actions to attempt to support continued policy use (usually Expand Economy to make more Econ to pay for things). Changing policies normally requires a Secondary action, but if war is declared can freely switch to the Defence or Offence Policy if so desired. Also, if a Policy runs out of necessary resources it can also be changed during the midterm for free, if desired.
So Megaproject Support grants us two additional Main actions for the project as Law upgrades one Secondary and Policy fuses the remaining two Secondaries.
Which means without a Double Main we can get 3 actions worth of progress(which should be the base for Gardens). With a Double Main we can get 4(which should be the base for Saltern, though we'd need to drop Province/Restore Order options)
Ehh... I'm pretty sure that anyone living close to a river already bathes/pisses/shits in the river. It is an extremely convenient way of both getting "clean" water and removing the waste generated by a society.
My main concern is that if we share the secret of hygiene with the metal workers, they are going to inevitably spread it to other cultures. Either we need to keep it completely secret, or we should at least grab the +1 stability from sharing it with our immediate neighbors. Splitting the difference gets us the worst of both worlds.
Hygiene isn't an easy concept to convey. Note that most cultures don't have a hygiene taboo at all in this era, it takes really ridiculous efforts to maintain enough water that it's practical.
What's likely to happen is:
-Metal Workers
--Ignore water hygiene element. They've always done it this way.
--Accept boiling water. Just put a copper kettle on the smelters. it costs them little to do this.
--Accept Salt potion. They can make it themselves, and salted water alone helps substantially with relieving anything that makes you sweat, shit or piss.
--Maybe Honey potion. Not sure if this is a scam, because the main source of Honey is the guy saying this is a magic potion.
Whoever they tell about it might pick up boiling water and using
-Northern Nomads
--Accept water hygiene element. Due to being nomadic, it doesn't take as much effort to avoid polluting water, rather than having to go rearrange all your infrastructure expensively(remember, we would find if far harder to do this if we didn't have universally Organized settlements)
--Ignore boiling water. Wood is too precious to waste on burning WATER of all things. Silly Ymirri.
--Maybe Salt potion. It helps, but the supply depends on trade.
--Maybe not Honey potion. Not sure if this is a scam, because the main source of Honey is the guy saying this is a magic potion.
Whoever they talk to will just carry the important lesson of not pissing where you are drinking. This is good for us, because they are upriver on the Black River, and so if they catch Cholera, we're going to be affected.
-Thunder Horse
--Accept water hygiene element. It's difficult because of the need to rearrange their herds, but their region lacks much water, so protecting it's quality is worth the cost.
--Ignore boiling water. Wood is too precious to waste on burning WATER of all things. Silly Ymirri.
--Maybe not Salt potion. Not sure if this is a scam, because the main source of Salt is the guy saying this is a magic potion.
--Maybe not Honey potion. Not sure if this is a scam, because the main source of Honey is the guy saying this is a magic potion.
Whoever they talk to will just carry the important lesson of not pissing where you are drinking. But they might decide that this will be a way to poison the shit out of the Dead Priests so the bastards shit themselves to death.
-Dead Priest
--Maybe water hygiene element. They'd need to rearrange how everything works on the lowlands to avoid latrines getting flooded during wet season, and everyone upriver is doing it anyway. Why bother?
--Ignore boiling water. Wood is too precious to waste on burning WATER of all things. Silly Ymirri.
--Maybe Salt potion. They have their own sources after all.
--What's honey?
Whoever they talk to will not get anything but Oppression.
-Highland Kingdom
--Maybe water hygiene element. They'd need to rearrange how everything works on the lowlands to avoid latrines getting flooded during wet season, but this is not a problem to implement near their capital in the hills.
--Ignore boiling water. Wood is too precious to waste on burning WATER of all things. Silly Ymirri.
--Maybe Salt potion. They have their own sources after all.
--Maybe not Honey potion. Not sure if this is a scam, because the main source of Honey is the guy saying this is a magic potion.
So whoever they talk to will know that salt is good, and keeping water clean is good.
Remember, Stone Age telephone game. Technology does not spread out indefinitely. It spreads a short distance before it's distorted beyond usability unless you have it in writing.
And it turns out, most factions won't HAVE our level of writing.