Is a gym with two active defenders unbeatable?

Nope, you get a messege that they gym is under attack and no mon can be placed within 10 mins.

There is an internal cooldown to place a mon in a gym that can't be avoided unless two or more players enter their mon box at the same time and then add the mon, it is the only workaround, but this does not gets around the cooldown when someone else is taking down the gym, only when it is completely blank and you add new mon on it or there are already some other team mates inside and you want to add multiple defenders at the same time.

When the gym is under attack, I believe you cannot add defenders. Also the effect of feeding berries decreases fast and there is a cool down period after you feed a certain number of berries. If an attacker is really persistent, he can wear down two active defenders since their only resort is feeding berries, which is not very useful in my opinion.

Even though people get the "gym under attack" message, I still had two Mystic players sneak in extra defenders when I was trying to claim a gym last night.... really frustrating, not sure how they did it. Maybe because it was turning over constantly the 10 min thing was bypassed?

Just had his today, cleaning a gym with four wrong colored mons in. One can still fill the slots which were free when the attack started, but can not replace mons which were kicked out. The opponent also fed berries with multiple accounts, the solution was to only attack the first mon over and over again till it drops out because he can't feed fast enough (There was an egg with a raid boss about to hatch and apparently he hoped to be saved by the bell, but I beat him just one second before the egg hatched and LMAO)

I've already stalled a gym.
Every time the defenders got to the last bit of motivation, I fed a Golden Razz to them.
After around 1 hour of doing this, the attacker gave up.

If the attacker is slow, there's enough time for the berries cooldown to wear off before the next "meal" and you can keep the defenders up.

Only golden razz can do that, tho.