The splitting advice is correct in theory; it can become instant-release and thus briefly stronger, even dangerous. But in this case I trust my belief over science that trying 1 % - 5 % first is always the safer option. Splitting a slow-release by 50 % - that might cause this problem, yes.
There is also the theoretical possibility that the active component(s) are not evenly distributed. Even a split marker is supposedly not safe, only instructions that say so. But - doubt
Take into account that Modafinil is very unsafe in combination with many other drugs, such as all benzos. I don't know how much time you need to be safe, but I'd wait at least a whole day (48 hours after taking Modafinil) before using something that is definitely unsafe with it.
Did you also get it through a EU prescription from a semi-shady, but legal site?