Showing and detecting interest
Seems like in the NT world, interest is often defined as sometimes initiating, and it is tightly coupled to conversations too. But what if you are a shy ND, or have a love interest in a shy ND? Surely, those rules from NTs will be no good for you!
However, there are simple and more generic rules for how interest is expressed that works even between two shy NDs (well, maybe it doesn't work for extroverted NTs, but that is of minor importance). The rule can be expressed in simple terms: Somebody is showing interest when they use whatever means are available for reciprocating regardless if it is verbal or nonverbal. So, if you are two NDs that don't dare to move beyond simple eye contact, you can show mutual interest by reciprocating with eye contact only. If you are not meeting each other naturally, you need to add some effort to meet, which can be done in completely nonverbal ways. Another way is to play around online. It's amazing how much you can do just by finding your love interest on Facebook, and I don't mean sending a friend request and starting a PM discussion.
I think in any love interests between NDs, it's very important to first make sure there is mutual interest. As the definition of interest now is no longer initiating contact or asking for a date, but using whatever means are available to reciprocate, it becomes a lot easier to do. The shy guy no longer needs to approach a girl and ask her for a date (and with a high probability of getting rejected).
The next rule is that contact should always remain constant or increase. Decreasing contact is a good clue that somebody has lost interest, so if you do that with a love interest, you are communicating "I'm no longer interested in you". That means you should NOT start with a session of intense contact, as that always means you need to decrease contact later. Make sure you start with a level that is sustainable long-term. Another reason why NDs should not start with intense contact (especially not verbal), is that their attachment will jump-start to highly unhealthy levels, which will give them a lot of hearth-ache when it doesn't work out.
But, surely, very one-sided communication is unhealthy no matter what? Not as long as there is reciprocation. The guy (or girl) could monolog completely with their love-interest, but as long as there is some kind of reciprocation, it still works and is healthy. That's partly because reciprocation to monolog typically is a way to show selective interest in what is said, and so can be used to get the opinions of a love interest that is completely nonverbal. It's a lot like you can figure out what people are thinking about just be asking smart questions that are answered with yes or no.