Do you think it is too much to have more than one romantic-interest story plot/subplot going on in one novel?
I am writing a novel and considering having two completely separate romantic stories going on - two couples, four people - but does that simply have the effect of making the audience fail to adequately care for or follow either one?
Perhaps one romantic plot/subplot is all readers can really keep up with in a novel and still retain interest?
Just for clarification, these two couples have nothing to do with each other; they never meet each other or know of each other’s existence.
In other words, it’s not like there is any love triangle scenario.
Sure. No hard rules in writing. Try it, and if it’s not working, you can change it later.
I don’t think having two romantic plots is a problem in and of itself. Several of Jane Austen’s novels have multiple romantic plots, for example. However, having two totally unrelated plots (romantic or not) featuring characters who never meet or even learn about each other strikes me as a questionable choice. There are authors who could pull this off, but if you don’t have a very compelling reason why these two stories should be in the same novel then you may be better off saving one of them for another project.
It might work. It might not work. It’s hard to know without more information. Is there anything at all tying the two couples together, even if they never meet? A setting, a theme, a message? If there’s absolutely nothing, why are they in the same book?
“There are nine and sixty ways of constructing tribal lays…
And every single one of them is right!”
As my old creative writing prof used to say, “Of course it’s okay…if you do it right”
The parallels between the two courtships could be an integral part of the story, a kind of “Tale of Two Cities” parallel construction. You could use each relationship to shed light on the other.
Some call it complexity, and some call it “richness!”
Sounds okay to me but I’d want to make their interactions different enough so as not to irritate the reader because of getting them mixed up. Example: one couple is playful and uses short sentences while the other couple is more intense and wordy. Or any combination, but be consistent.
Is one or both of the romances the main plot, or are they both B plots to something else? Is the story centered around characters, or around events? Is your point of view static, or does it change from chapter to chapter?
There’s no way to judge without reading it.
In Our Mutual Friend, Dickens has two love stories. He pulls it off. In a thousand pages 
I read a book once with two couples and halfway through the author switched the names of the women.
And then switched back later.
So, I’d suggest that if you find you can’t remember who is who neither will your readers.