I realize its been a month since you posed this question but yeah there's a way to do it. I forewarn you its a bit of work.
Let's assume its a lake with two rivers entering it. We'll call it Lake Boeing, with the Cessna River entering it from the north and River Piper entering from the south. Boeing is a big circular lake. So you have your poly for the lake. Copy that twice for the two shorelines you're going to need to enter. Now, name one of the East and the other West, for the ease of keeping track of. The Boeing poly doesnt need to be around right now, so uncheck it in GE so it goes away. Starting with West, delete all put one point (it doesnt matter which) east of the west bank of the Cessna and Piper rivers. Then drag that eastern point really far east. I like to drag it far enough that its longitude is different, for reasons that will be easier to see later. Now your poly looks like (> sort of. Save it and load it in FSX_KML.
Now, here is where things get crazy. One of two things can happen. One is the nice thing happens: the point that's really far east (and you can easily tell since its the only one with -86 for lon instead of -87) is also point #1 and point #50 in the vertex list. If that's the case, convert to it to a line, which deletes point #50, giving you something like this (\. Then you just delete point #1 and its all over. Repeat with the other one, except dragging that rogue point really far west. Reload them into GE to put with the rest of your project, or whatever you want to do with your new shoreline KMLs, depending on how you personally do things.
But normally, things get ugly. In fact, I know I'm going to tell you the wrong thing to do even though I spent Monday night doing this exact process. Make your ice cream cone looking shoreline polys (the (> one) and save them. But lo, you looked at the vertex list and the far east point is #25! If you convert it to a line it'll delete the wrong point! But all is not yet lost. Don't convert it to a line yet. Close FSX_KML and get out notepad because we have to go under the hood.
Load your KML into notepad (or some other notepad like program, that I assume can open KMLs). You'll see a lot of scary looking information but eventually you'll see the coordinate list. And you'll know it when you see it, even if you're a computer dunce. What you'll want to do now is go to the end of the coordinate list. Delete the LAST POINT and nothing else right now. That is, the last XXX,YYY,ZZZ so don't get overzealous. Now scour the coordinate list for the far east point (now you see why I made one have a different longitude!) Copy and delete all the coordinates that came before that point (in this example, that's 24 points). Paste those 24 points at the end of the coordinate list, so the tip of the ice cream cone is #1. Then copy point #1 and paste it at the end, so points #1 and #50 are the far east point. Save it.
Then load it in FSX_KML. Now when you convert it to a line, and it deletes point #50 and you delete point #1, you're left with ( like you want. Repeat it with the other and you'll go from having 0 to (><) to ( ) like you wanted. Make as many ice cream cone fragments as you need for your rivers or whatever you need. I know this seems like a lot to do, but once you get the hang of it its not too bad. I could have done two in the time it took to type how to do it. I did six for my project and it didnt take much time after I figured out how to do it.
I pass along my knowledge, despite the month since the question and now if anyone else needs the help they know how too. I could probably provide pictures if I needed to.
