Join Date: Jul 2006
Location: Ft. Collins, CO
The stem plants you have can be planted 1 by 1 and will soon form their own roots. They're just cuttings from the tops of parent plants, but will establish themselves with time. For the moment, though, I would refrain from cutting them in half, as they are already stressed, and shouldn't be put under any more duress. When they grow and are ready to be trimmed, then feel free to propogate them.
On a side note, that grassy plant you have on the side is called Mondo, which happens to be a semi-aquatic. Unfortunately, it will soon die and rot in your aquarium. You should cosider replacing it with a true aquatic grass like hairgrass or micro sword. Good luck!!!