Two twin mattresses do not make a king. Twin mattresses measure 38 inches wide by 75 inches long, while a king mattress is 76 inches wide by 80 inches long. The difference of 5” would mean that two twins would be shorter than a king bed.
If you want to make a king sized mattress, you would need two twin XL mattresses. These beds measure 38” x 80”, so two of them have the identical dimensions (76” x 80”) as a king.
Pushing two twin XL mattresses together is an excellent solution for many couples. The separate surfaces mean zero motion transfer. Plus, you and your partner can each choose your own mattress. However, this setup isn’t ideal for adult activities, or if you share your bed with more than one person or pet.