So here's how to do it.
Buy more books
I'm partial to ordering four or five discount books at a time from the Book Depository or Amazon. The problem when they all arrive at once is simple: I don't know where to start. If you're a book lover, you'll know this feeling is the same of that of a fashionista - it's like looking at a wardrobe full of beautiful clothes and not wanting to neglect any of them. So when you buy more books - like when you buy more clothes - the impetus is on you to enjoy them. The fastest way to do that is to read more than one at a time. And then - bonus! - you can buy more books.
Choose different literary worlds
The easiest way to read more than one book at a time is to chose from completely different genres. With different characters, settings, themes, and writing styles, it's not hard to differentiate what story you're involved in. You'd never confuse what happened on one night's episode of The Handmaid's Tale with RuPaul's Drag Race's latest, so give your brain some credit. It has ability to separate different literary worlds.
See reading as an entertainment choice
Like watching a film or going out with mates, reading is a choice we make for enjoyment. It's supposed to be fun, engaging, and a relief from the stresses of real life. In order for it to always feel like entertainment (rather than a chore or a commitment) variation can be key. Sometimes I'll read half a book, then not pick it up again for six weeks because I've found something more captivating. I'll return to it eventually when the mood has struck.
There are books for work, and books for play
A lot of my reading consists of stuff you might call "professional development". Biographies, memoirs, business books... we read them because they might give us ideas for our careers and our futures. The problem with this? It makes reading feel like office time. Nobody wants to feel like they're still working when they're curled up in bed at 10.45pm. In order to stay engaged with a "book for work", have a "book for play" as well. Something completely out of your real life realm that serves as a break from reality.
Use a light book to make a heavy read easier
This is the main reason I tackle multiple reads at a time. If I actually want to finish it, I cannot read Ulysses night after night: it's just too drawn-out and dreary. Also, a lot of books of comparable length start off too slowly that I almost give up. Having a light book to switch things up - or even a magazine containing long-form journalism - helps you stay on track so you don't get fatigued.