Bob open a 1BTC channel with Alice and send her 0.5BTC. If Alice wants to send Bob 0.5BTC may she uses the same channel or it is just onedirection channel and she has to open another channel which is in direction Alice->Bob and use this for paying Bob.
What you need to understand is that LN is a kind of settlement. So, yes. Alice can send Bob and vice versa with one channel. But there are some sort of restrictions on receiving fund. If you have started a channel with 1 BTC, as long as you don't send any fund, you can't receive any too. You can only receive fund as the same amount which you sent although it can be increased in some way.