Yes, you can buy two split queen mattresses that are the same height but have different firmness levels. Split queen mattresses are designed to be used with adjustable bed frames (but do not have to be), which allow each side of the bed to be adjusted independently. This means that you and your partner can have different firmness preferences and still enjoy a comfortable night's sleep.
When shopping for split queen mattresses with different firmness levels, make sure to specify your preferences to the mattress retailer or manufacturer. They should be able to provide you with options that meet your specific needs. Additionally, it's always a good idea to try out the mattresses in a store like Nine Clouds Beds in Mississauga where every mattress can be made in split queen 30" by 80" size.