Has your job been covered with a full time member of staff? If so, the main impact on your employer's business/your job/colleagues is that if you return only 2 days/week, there is no-one doing your job for the other three days, unless they recruit someone else to cover those days.
The impact of that depends on what kind of work you are doing and how easily it can be broken up. Can you look after a part of your own workload on those days when you are in, and people can wait for you to be in the office to deal with things?
If work needs to be dealt with more urgently, then you would presumably be looking at a job-share. I would be thinking about how you would ensure continuity, so that you and the other sharer could work effectively on the same tasks. Would there need to be a hand-over in person (i.e. you would have to overlap hours), or could you leave notes, e-mails, or whatever? How much of your time would this take? (Bear in mind this would be happening twice a week, to hand work over and back again.)
If you have contact with other departments/people outside the company, again how would you ensure that they can still deal with you effectively, if your role is split between two people?
I totally agree with lljkk, it would be a good plan to talk this over with your line manager before submitting your application.
I am currently on mat leave after having my second dc. I went back from first mat leave for just a few months full time before going on second mat leave. I would like to request just 2 days as that is all I need to work to cover bills etc and don't want to be away from dcs any longer. I was working full time - 5 days. I have looked at my flexible working application form and it asks how my proposed working hours will affect My job and how can I overcome this and it asks How it will affect my colleagues and how I can overcome this. Tbh I really do not know what to put and our job roles are always changing and I am not really sure exactly what I will be doing when I go back as it was nearly 2 years ago when I went off on first mat leave and I know things have really changed. I don't mind what do to be honest. Not sure whether it's best to ask for job share or not to mention that on application form and just ask for part time hours. But I am not sure how to justify it to them.