If you are applying for a job and tell me that you can do TWO jobs that I'm hiring for, combining them into one, and I tell you that's really not possible because the workload of each is a full-time job on its own, you should believe me.
I know that job-seekers are often told that they should propose new and innovative solutions to employers' problems, and that's great -- but if the employer assures you that you're off-base about something internal like workload, they probably know what they're talking about. Unless you've seen reason to believe them incompetent, you should believe what they say.
It's irritating when a candidate keeps insisting that they can do two jobs as one person, after I've already explained that won't work. Here's why: I am competent. I hire competent, efficient people who produce at very high levels. We don't have slackers sitting around with little to do. We have the opposite problem: too much work. I know this because I manage the organization, for the love of god. I monitor workload levels like teen girls monitor Robert Pattinson.
By all means, make the suggestion originally if you want to. After all, many places are mismanaged and maybe do have three people doing a job that could be done by one person. But I'm not that manager, and I don't operate that way -- so please don't persist when I explain to you why it's unworkable. When you insist that you are a better of judge of something very hard to perceive from the outside like workload, despite what I tell you, it's hard to think you're not being a naive and/or presumptuous d-bag. Cut it out.