This is something that I think completely depends on the writer. Personally, I can write multiple stories at once and keep my ideas and universes separated from one another, but there might be some people who find the challenge to be tricky. What I think you should do is figure out what kind of author you are: are you better at dedicating your time to two or more projects at once, or is it better for you to channel your energy into one story?
If you're the latter and you have more than one idea, that's okay. Stories don't have to be written right away. Jot down your ideas and save them, and even leave room for edit so that if something comes up, you can quickly write it down so you don't forget.
The more you write, the more you will get better. Practice makes perfect. If I was the latter and had multiple story ideas, I would pick the one I'm least interested in and start it. That way, I can use it to practice so I can put my best work toward the storyline I'm more leaning toward. If I run into a writing slump, I set it aside to come back to another day. Nobody is rushing you, and you're free to write at your own pace. If you take a week to come back to a story, then so be it. If you take a month to get back to a story, then so be it. If you take a year to get back to a story, then so be it.
Meanwhile, if you're finding yourself thinking of scenes that you wish to write but don't fit the storyline, why not slip aside and write a separate story? Personally, I have a pairing in my story that I love to death, and I wanted to write some explicit scenes too, but I felt it wouldn't fit in the storyline. So, I opened a new doc and wrote a separate story with the same characters in the same universe. There are tons of authors who write side stories, spin offs, or extras. Save them for that!
Take the danmei writer Priest for example. In Qi Ye, she originally created Zhou Zishu as a side character to help Jing Beiyuan, the main character. However, some brain rotting went on in her head. Before she even finished Qi Ye, Priest started writing Tian Ya Ke, where Zhou Zishu was the main character in the same set universe some years later. She still kept these two series separated. At the end of Tian Ya Ke, there is an extra for her pairing from Qi Ye. Another example is the danmei writer Meng Xi Shi. In Thousand Autumns, Shen Qiao and Yan Wushi are the main characters. She later wrote Peerless, and the pairing from Thousand Autumns makes a small appearance, but she still keeps focus on her main characters of Peerless.
The main idea is to keep the two series separated, even if they are in the same universe. Depending on how you organize your thoughts will let you know which one is better: is it a better idea to do what Priest and Meng Xi Shi did and keep the same universe, or is it a better idea to make sure these two ideas are completely different so you don't mix up events, situations, characters, etc?