I have an inkbird 306t on one of my 180's. There are two sockets for two heaters and both sockets are for heating only. I've just checked mine now to see if both heaters come on together when I raise the set temp. And yes they do. Also the "work 1" light came on. And when I decreased the set temp both heaters went back off together and the "work 1" light went off. To be honest I'm not entirely sure what the "work 2" light is for, nor do the instructions seem to shed any light on it. Maybe the "work 2" function has something to do with the units ability to do seperate day and night temps but i don't use this function so I could be wrong.
So if what inkbird are saying is correct (that the heaters should flip flop between sockets and NOT come on together), then mine must be broke!!!
Just some extra info for you. I've got two 180's and used to have inkbird 306T's on them both. However, I had nothing but problems with one of my inkbirds on one tank so I bought another to replace it, and not long after the replacement started to play up. So I took the replacement out, i'll not buy another inkbird now. My other 180 still has an inkbird in there and that one seems to be doing OK (though if what inkbird are saying is correct, then that one's broke too!!!).
Good luck with yours.