I know that splitting your core was created by the rival of the first Empty Palm-user. I just do not think that Simon and the rival are the only people in the world who have thought of this. Surely their must be others. Also, can one keep splitting their core repeatedly. I can imagine how strong Lindon could he be if he had multiple paths in his body. I'm still unclear if splitting your core doubles your madra and if it has a cost. Another thing that comes to mind is can Lindon upgrade one of his cores to jade while still having his other core at the copper-level?
This will be explored in Blackflame, but I don't really consider it a spoiler, as it deals with mechanics we've already seen in the books.
If you're especially allergic to SPOILERS, stop reading now.
No, the Heart of Twin Stars is not the only technique in the world for splitting cores. Others have done it, and I expect we'll run into them.
Yes, you can split your core repeatedly.
Splitting your core does not double your madra, and there's a very slight loss in energy when you split. So if you had 100 MP before the split, now you have two sources of 49 MP each. The other 2 MP are lost in the process.
He can upgrade one core without the other. At the end of Soulsmith, one of his cores is Iron and the other Copper. But he's considered an Iron, because his body was upgraded.
Here's something else that will be dealt with in Blackflame: an Iron core split into two doesn't make two Copper cores, it makes two very small Iron cores. Your stage of advancement is based on the quality of your madra, not the amount.