Multiple accounts and sanctions.

I think Niantic staff 'll give you a Warning first,
Like the warning who some people get for the use of bots and Hacks....and after that if they didn't stop,Will be banned permanently.

I hadn't see any post or case like your. But i think that some points:
1: Till the date Niantic did not menction about sanctions for multiple accounts. It only mentioned in the Terms of Use but in future, i pretty sure they'll do something
2: "in Future" they'll recommend us to only use one account and delete the secondary/rys. That option exists in the official site yet.
3: Niantics will send a Warning for first, did not do instantly Bans for obviously reasons.
4 what kind of Ban? Permabans and Multiple Accounts bans
5: Why not Ban only the secondarys account? Cause Niantic can't know or don't care what account are the primary.
6: When did you think it'll be the "Future"?
When Niantics'll release the Pokemon Trading.

I don't wanna sound like a Niantic crusader but we have to know that the company do not like this practise and if this practic is not controlled could broke the game when the tradings will be habilites. Just think about catch 4 snorlax with 4 differents account or use your yellow account to put down de 10th pkmon of a red team and after put your pokemon with your red account. Where is the sense? Where is the fun?
P.D. Sorry for some writing Mistakes my first language is spanish xD

Of course you're going to choose 2 teams if you're cheating. Use the first account to place a weak pokemon in a gym. Level the gym up to level 10 in a matter of minutes. Use the second account to take your weak pokemon out, so you can place one of your strongest pokemon in the gym. I'm glad Niantic might be doing something about it.

I don't have an answer, but why do you play with 2 accts? Isn't that a hassle?

Niantic' official trainer guidelines specifically prohibits using multiple accounts:

No cheating.

Don’t do it. Play fair. Pokémon GO is meant to be played on a mobile device and get you outside to explore your world! Methods of cheating, unfortunately, are limited only by cheaters’ imaginations, but include at a minimum the following: using modified or unofficial software; playing with multiple accounts (one account per player, please); sharing accounts; using tools or techniques to alter or falsify your location; or selling/trading accounts.

https://support.pokemongo.nianticlabs.com/hc/en-us/articles/221993967

For what it's worth, I have not heard of any instance of account banned specifically under that rule.

It's against their ToS so if you want to consider yourself a cheater, go ahead.

it is you who kill the weakest pokemon arenas 10, and replaces it with yours on your other account?

Don´t be afraid. My area is full of multiaccounters and Niantic don´t care.

You can cheat like hell with no problem