Turning comments into an answer:
Include the full amount as the costs of the trip in each application. In the subsequent questions asking how much does the applicant pay and whether anybody else is paying for the trip and why, each applicant should specify the details and explain them.
The application also asks about travelling companions. To provide the ECO with the full picture, decide who is the ’primary’ applicant, complete their application first and make a note of the GWF reference number (assigned when an application is submitted). Then link the two applications by including the ‘primary applicant’s’ GWF number in the second application.
As Augustine of Hippo commmented, you could apply completely separately and just put down reasonable numbers that can be supported by your salary. However, IMHO being completely transparent about your travel plans is always a good idea and could even help avoid any possibility of a one application refused / one granted scenario.
As an aside, the UK does not require proof of travel/hotel reservations as part of submitting a visitor visa application (unless transiting). See Documents you should not use as evidence