It’s not a problem to add them at the same time, but do measure your current salt level with a reliable test before you add the new salt. Chlorine and some of the other chemicals that we routinely use will add salt to the pool and if your pool is several years old it’s quite likely that it already has a significant amount of salt in the water. If you don’t measure and assume it has 0 salt you will likely overshoot and need to drain some water to remove the extra salt.