Yes you can just place them into a single action list, or they can have their project, doesn't matter.
The notion you cannot implement more than one habit at a time has no basis in anything. We implement multiple habits at once all the time. Get a new job. You will be implementing likely 100 habits or more all at once. Find yourself in a new romance; again 100s of new habits. Your spouse dies; 1000 new habits. Get sober, 1000 new habits. Most of the ideas people peddle in self-improvement circles are just nonsense.
Now what people really struggle with is developing habits they don't want to do. So don't do them. That's the easiest solution and it always works.
If you think you would want to implement those habits, then you must know that they are good, then there will be no resistance at all. This is the solution and it's about as old as western civilization itself. If you know the good, you will do the good nearly effortlessly. Most people just think they know the good and try to force themselves to achieve it. Having thoughts about good is not the same as knowing the good (episteme vs gnosis approximately). So come to know the good. This is why something like doing things which approximate the new possible habit can be helpful. We get to see if it is truly good or not. So taking a walk for 10 minutes can give us an understanding whether running 20 minutes is truly good. You might quickly find out all the paths around you are awful and drivers aggressive. Welp, running 20 minutes is no longer as good as you thought and you know it.
The root cause of people being confused about such things is the rise of voluntarism in the West. If you believe your will is anterior to your intellect, it will be a struggle. You see this in all the early self-help literature and we haven't changed out tack even tho have had 150 years of proving willpower means nothing.
When someone is convinced something is good, the habits flow easily. Again take the new job. People are convinced it's good; it requires a ton of effort to get a new job usually. And then they change their wake up time, driving patterns, where they sit during the day, new names to remember, new places to have lunch, etc. You could list probably 1000 new habits and it works fine.
Want to run 20 minutes a day? And you struggle to do it? You don't know it's good (and it probably isn't), so you struggle. You can hack yourself into it maybe. But with all the failed New Year's resolutions, we see that people are very bad at doing what they don't know is good. Very bad at it.
And this ignores your own habitus. We just think actions are atomic self-contained and isolated things that you can just mix and match. You can't without doing violence. Everyone lives in a habitus governed by a given over-arching telos. If the new action does not make sense within that habitus / telos, it will be harder to manage than one which naturally is given to such a habitus or telos. This is why something like running 20 minutes a day can be very different if you change where you live, your friends, your spouse, etc. But those are pretty radical changes to just do some activity which is in the main is just miserable. And it explains why people have a rather extraordinary capacity to change when their habitus is changed: thrown in prison, survived tragedy, went to war, entered puberty, had a religious experience.
So yeah, when you start messing around with your habitus, there will be a high cost. But it can be worth it if it achieves a greater proximate good. But usually most people should just stop with all the self-improvement. It's dubious any of it is an improvement. It's often just another bolted action usually driven by some fad.
GTD tries to address the above via its horizons of focus, but it doesn't force anyone to consider whether they are teleologically coherent.You just add whatever you want. Welp, that will be a struggle. Thankfully most people just do the exercise as descriptive rather than prescriptive.
So I would return to those horizons of focus if struggling with a change and ask myself, is this a real thing? Do I think this is good? Do I know it is good? It is coherent with my pattern of living since I was born? Is it coherent with the world around me including those I live with? If not . . . It's going to be hell to pay. Which can be a fine a price to pay, just be aware of the cost.
If a new habit isn't a easy as calling, texting, seeing a budding love interest, or less romantically, as easy as starting a new job, then we have to dig in a little deeper about what we think we are trying to do.
This is why stuff like the book mentioned above, Tiny Habits, is dangerous frankly. As the previous poster said, 5 squats while brewing coffee . . . In what world does this make sense? And if one exists in which it does, do we want to live in it? Take the categorical imperative lol. Do we want everyone doing 5 squats when they brew coffee? It's just a list of ways to create a radically disintegrated life. It ignores telos. What end are we made for? It destroys habitus. Maybe we should burn down the world. But you better ask if that makes sense. I can assure you a world where everyone is following a unique list of ways to better themselves has had a name for a long time and it's called hell. Was great-grandma doing squats while brewing coffee? If not, we had better pause long and ask why we think we should be.
More than you asked for but I find self-help stuff incredibly interesting even if in the end it's mainly a recipe for civilizational disaster (look out the window).
But yeah a single action list or a project will do!