I do not think that is possible on Shopify, maybe someone can correct me. Your best bet would be to add a new page template to your main theme. You can do this by going to templates > add a new template > create a new template for > page then you can call it something like landing_page. From there copy the landing page contents from your second theme into that file. Then create a page as you normally would and make sure you select the page.landing_page file as the template. After that I would create a redirect from /landing to /pages/<name of your landing page>. This can be done from online store > navigation > url redirects. Now when someone goes to your site /landing it will go to that page.
@michaelPeterson Thank you a lot for the answer! Could you please explain how I can copy the second theme (templates, sections, etc.) as a page? (It's about "From there copy the landing page contents from your second theme into that file.")
Ok so I miss understood what you wanted for the second theme. I thought you had a single page from that second theme that you wanted to display to customers. That would be possible if you just copied the single liquid file associated with the page and pasted it into another page template. What you want is not possible you can't copy all the template files and paste them into your main theme and have a second store running on your store. The only thing I can think of is if you used an iframe to display the other store in it's entirety here is a good resource on iframes: https://www.w3schools.com/html/html_iframe.asp now if you went this route you would have to have your second store running and all setup like a normal shopify store. You would then setup an iframe on a new page you created that links to that store. This would also mean you would need to pay for two Shopify stores and maintain the inventory/products for two shopify stores.
I am wondering though why do you need a second theme/store to be running on your main store? What exactly do you need from that second theme? It is possible that whatever you need this second theme for can actually be achieved in a much easier way.