Multiple plots should belong together
All plots in a story need to be able to answer the question of why they are in the story and why they are there together.
This basically means that there needs to be some connection between the plots. One causes the other. One provides information to the other. Or one influences the other. Or, of course, even better; they influence each other.
Thematic connections also work. For instance, showing the same/or a similar problem being dealt with in different ways, or showing opposites, etc. All in order to show the theme/message.
You would usually decide what plots should go into the story in two places:
- Before you write the story while outlining it or planning to write it
- After you've written the first draft while editing it
On both these occasions, you can brainstorm ways to connect the plots even more. This could be done using setting, characters, events, etc.
If you're halfway through your first draft, I don't recommend going too deep into this work just now. Chances are, at this point you don't really know your story and it would benefit most if you finished the first draft and then took a step back to figure out what needs to be changed.
So, the rest of this answer is probably going to benefit a story in the planning stage (1) or a manuscript finished at the first draft level (2) most.
Use the message/theme
In your case maybe the message is that action without information is going to fail. Your secondary-research plot shows a character getting all the info before acting while the primary plot shows people acting without having all the information. You might add some inciting event that will make the characters in both plots start acting (anything from something they both read in the news to one requiring the assistance of the other).
Or, you could flip it. Maybe the message is that thinking without action is never going to make change happen. The characters of the main plot force the characters of the secondary plot to get out of their libraries and apply their knowledge in the real world.
Or, combine them. The message being acting without thinking is just as fruitless as thinking without acting: The main plot and the secondary plot depend on each other and the characters in each plot will teach each other to think before acting or not just hide behind all that knowledge but also act on it.
Maybe the main plot isn't the main plot?
You've noticed that your main plot seems thin. Maybe it's not the main plot? Maybe the secondary plot is the main plot and the main plot is secondary?
Maybe you are trying to force your story into an archetypal format for mysteries (maybe even action adventures), but that isn't working out entirely?
Writing to a format, for instance, crime or adventure can be a good thing for sales and popularity of the novel. It's "McDonald's." You know what you get and you get the same quality every time. Safe and predictable, and a good time reading it. So your audience might be more likely to purchase something that feels well packaged in that archetypal/genre package.
However, your story is dependent on you as an author as well. If you find yourself having trouble fitting your story into that format, perhaps you shouldn't? Who said a story from the research angle wouldn't be successful? Especially if the "action"-plot came crashing into this researcher's safe and predictable life and caused havoc for them. (Hm, isn't that a bit like "The Da Vinci Code?")
External plot vs internal plot
I'd also like to mention Witness. (Check it out if you haven't.)
What's the main plot here? Amish romance or a bad cop trying to silence a witness?
The two plots are connected, but the Amish plot takes the main stage of the story for most of the second act and the bad-cop plot only really enters the story in the beginning and end.
What's happening here is a very distinctive division of the story into its (usually) two "sides of the same coin:" The external, action-driven plot, and the internal character-driven plot.
The external plot is about external events, actions, threats, ticking bombs, and bullets flying in the air (or maybe words flying in the air, or betrayals... all depending on the genre, etc.)
The internal plot is about character change. It's not always present in all types of genres, and in some genres, it's the central plot.
These two usually go hand in hand (in Witness they are both carried by the main character, John Book).
Maybe your story's two plots are one internal and the other external? Or maybe your story could be changed to make that happen?
To make that work, you usually need to find a character that can be in both. For instance, the primary plot is forcing the character in the secondary plot into a change arc (inner change).
Thin and thick
If by "thin" you mean one plot has fewer words than the other then that's ok. But I feel you mean one plot is of lower quality or not as interesting or not as attractive than the other.
You should try to change your story so you have no "thin" parts according to the second definition above. If you can see it's bad, boring, etc, improve it or cut it.
Of course, there will be a point where you'll have to say enough is enough. A point where you won't be able to go further.
That's good and it's necessary or you'll never finish your first novel and never be able to shine more in your second...