I myself won’t work on two at once UNLESS you know what you’re doing, and have a very good schedule planned out. Dropping projects frequently is definitely normal for authors, but it is a bad practice(attitude wise), and shouldn’t be done often unless there’s a valid reason other than just laziness. I’m going to be very blunt here. Most often than not, if you feel like you’re making excuses, then you probably are. We subconsciously generate excuses to justify our actions, and if we can notice it on our own, then it probably is something that is that huge.
Alright, let’s look at it from this point of view. If you feel as if you want to start another story, what reason do you have that the second story won’t turn out like the first? Dropped because you have a better argument for the next object? It’s a fallacy that a lot of people fall into(myself included). It’s alright to drop projects out of a lack of inspiration or motivation, but as long as you know where you want to head in a project, it’s probably not a good choice to write something just because you suddenly got an inspiration.
I can’t really give any concrete advice for this situation, since it differs from individual to people. I can only offer an additional perspective. To be very, very, honest with you, the best way you have right now is to focus on a single work. If you consider yourself to be lazy, then fix it. Get the right attitude, or none of your work will succeed no matter what choice you take. We procrastinate once in awhile, sure, but jumping projects simply because it is troublesome is probably not a good idea.