Conventional wisdom on the boards states that when it comes to online game you should never send 2 messages in a row because it comes off as needy. But is this a rule that should be set in stone?
Think of it this way, when you are in a set in person you can see the girls body language and this allows you to calibrate when she doesn't react to you the right way. Online however, there is no body language, so the only calibration you can do is contextual.
For instance, let's say you have sent out a message to a girl and she responds fairly quickly. You think, "Perfect, with online game getting that initial response is the hardest part. Now I can work on attraction." So you start messaging back and forth with no long pauses and then you fuck up and send a lame message to which she doesn't reply. Should you stop messaging her because general convention says that you'll come across as needy? NO!
In most situations it is inappropriate to message a HB twice in a row without a reply, but when you screwed up and you know it then you need to adjust. Like I said earlier, if you were in set you would get immediate feedback from her body language, but here you need to be the judge of your own material.
But now I want to qualify. You are the worst judge of your own material. If she doesn't respond to you you are going to start feeling self-concious and automatically assume your material was shit. If you fall into this line of thinking you are better off not sending a second message.
In summary, I believe it's ok to send two messages in a row as long as it is contextually appropriate. If your opener gets a response but you fail to hook her then you can't freeze her out and expect a response in a day or two. Instead, send a second message that is more interesting than the first.
DO NOT call her out for not responding to your message. DO NOT use self-deprecating humor since you're trying to show high value at this point. DO NOT give your first message any credence in your next message.