Can I split a 100mg Lamictal in half?

Yes. I used to have to do that all the time... doesn't change a thing except the dosage.
FYI- Be careful not to leave it in your mouth too long when taking it. Leaves an icky after taste if you're not careful (you may have already learned this though).
BUT as i always say, make sure with your doctor that you're doing what you need to do. Don't rely on other people's experiences. Everyone's different. But how boring would this world be if we weren't? :)
Hope this helped!