Both women assumed the role on 1 November. Ryan and Biddle will each work three days a week, splitting the responsibilities of the role equally, while both becoming full members of the company's executive committee.
Ryan, who has been with the company for 14 years, had been the company's sole HR director before taking maternity leave, when Biddle stepped in to cover.
Having a strong working relationship with Biddle that spans 15 years, Ryan approached her about sharing the role when she returned.
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One of the benefits of having two HR directors, said Ryan, is that neither is spread too thin, and the team always has someone to go to.
Speaking to HR magazine, she said: "The guys on the team can actually have more of us rather than trying to be spread across more disciplines than we would normally have, and we're always that point of contact for those that we may not have direct reports for. The team has obviously been familiar with Fiona over the last year, so the transition has worked really well."
The company also benefits from twice the experience, said Biddle: "Having two heads on things is a big plus.
"It's always a challenging job, whether it's HR or any other profession at the top, but I think with Vicky's background and also me coming from a different organisation, and having lots of experience that I can bring to the role, having both our heads on big problems is really helpful."
Both are seeing improvements to their work/life balance by sharing the role.
Ryan added: "In my situation [returning from maternity leave], it's been amazing. It's about me having the opportunity to remain at the level that I was working at, but still raise a family – you can't really put a price on that. It's a fantastic way to get women back into the workplace."
Equally, it allows Biddle time for other responsibilities. She said: "I'm of the age where my family are adults at the moment, but I'm also a counsellor as well. This arrangement allows me to do more counselling in those two days that I'm not working here, which is great. I was squeezing that into evenings and work/life balance was a little bit off when I was covering, but I think there'll be a lot more balance now as well."
Ryan and Biddle said the move gives them the opportunity to set an example for other part-time workers who want to be able to set boundaries at work.
"As it's a job share, it's easier to set boundaries," Biddle said. "Unlike a lot of women who come back to work four days a week but then really work five days a week and just get paid for the four days. In our role, we trust each other implicitly and know that the other is doing the job while you're not there.
"The role modelling that we need to do in allowing part-time workers to set boundaries in order to really benefit from that work/life balance that they're seeking."
Ryan argued all businesses could embrace job sharing more regularly if they use the right approach.
She added: "I don't necessarily think that job sharing should be limited by the content of the role. There is a way to work around but it's about taking the time to understand how it could work.
"Whether it's a head of finance or head of HR, they're very differing roles, but if your job share is set up well, you've got the right relationship, you set your clear parameters and you've got the right team around you, it shouldn't necessarily have a difference from one profession to the next – it should work."