Hi treonn,
Post #8 here may be helpful and there is more feedback about split king mattresses in this topic as well.
Your mattress protector and sheets will keep the topper in position.
You can read some thoughts about buying a mattress/topper combination in post #2 here. Unless you can test a specific combination in person there can be some some additional uncertainty and risk involved with this approach because buying a topper that is a good “match” for both you and the mattress you purchase can sometimes be almost as difficult as buying a mattress itself. You may also find that it will be less costly to buy a mattress which already has the same materials inside the mattress as buying a separate topper and you may also lessen the risk of buying a base mattress that has some lower quality materials in the upper layers which may be subject to softening even with a topper on top of them.
Phoenix