When putting a split king bed together, you'll want to think of each side of the bed as its own entity. For example, if each side of your bed has a maximum weight capacity of 400 pounds, that means you can put 400 pounds of mattress and sleeper on each side of the bed.
Technically, this means your bed can take 800 pounds altogether. However, that 800 pounds needs to be somewhat evenly distributed across both sides of the bed, and neither side should exceed 400 pounds. The good news is twin XL mattresses are typically not all that heavy and should not exceed your side of the bed's capacity.