Honestly, I think having to get two separate degrees, certifications, and licenses is what is completely unjustified in their situation.
Since they are basically of one body, cannot independently do the job of two separate teachers, and were employed for one position, then it makes sense to get one salary.
The university, state, or BoE that required them to get separate degrees/certifications/licenses should reimburse them one degree/cert/license since they will always be required to be employed as one person.
That said, I wouldn’t bat an eye if they were paid two salaries. Their district should either pay for two certified teachers or petition that they be reimbursed for half their expenses to receive degrees and become certified and licensed.
BUT since that will never ever happen, pay them two salaries.