Will it cost more to have two people?

Maybe a little but what you gain is more significant.

As with any new hire there are costs. Now, the factor of two doesn't have to cost more per say - some job sharers do 2.5 days each, so the company is still paying 5 days salary vs. 6 days. In the case of 6 days, the company would pay approx. 1.2x the normal salary. Other benefits are normally based on the salary split and pro-rata. Equipment is a minute consideration, if it dare comes up. And if deskspace is tight - remote working is always a possibility. Either way the gain is significant for companies - See the Roleshare Guide for The 8 Benefits of Shared Roles.