Is this legal?

mysuni

can a company turn down a request by 2 possible redundancy candidates to do a job share and bring in a new person?

CFC

Yes they can, providing they have considered it and have concluded that a job share is unsuitable. Not all employers offer the possibility of job sharing and if your proposal is the first their general policy may be not to have job sharing, which is an adequate reason. In the same way not all companies have part time employees.
Have you sold the benefits - often employers just see the drawbacks?