I didn't understand if you had meant whether it hits two times or not, or is it good in competitive, so I will answer both of your questions from this replay and this replay.
Here in the first replay, Mega Kang can take a hit from Zacian-C's Stab (though it is wrecked by Close Combat) and is able to Counter it with Counter. And it shows that Zacian has been hit one time (if you observe clearly). So she can Counter two times. In the second replay, she does the same with Zamazenta and hits two times with same damage (51% and 49%), just like Seismic Toss as Swastik said. (Question no. 1 answered.) But she has faced 81% Damage from Zacian. Seeing so many Legends in Ubers who have way more Bulk than M-Kang, I say counter is decent, but isn't a very good choice overall.
Hope this helps :)