Ash here from the Shopify team. Welcome to the Shopify Community!
It sounds like you currently have multiple Shopify stores, and I understand you wish to add products to one and have those products also be on the other. I'm happy to help come up with some solutions! By default, each Shopify store will have its own separate product list. This means that if you want the same products on multiple stores, you will need to; add them to both stores, add them to one store and export/import them to the other, or use an application which syncs products between your stores. The first option of adding them to both stores is self explanatory, and will probably be the most time consuming. It's totally an option, but let's also explore the other options!
Adding the products to one store, and then exporting/importing them to the other:
Using this method, you'll start by adding all of the products to one of your stores as you normally would. Once you've completed adding the products, you can export your product list to a CSV file. Next, head over to your other shop, and you can import that CSV file to get the same products in both shops!
Using an application to sync products between stores:
There are a number of applications that can be used to sync data, such as products, between multiple Shopify stores. Here are a few options for you to check out, and here's a link to the Shopify App Store where you can explore some more options!
I hope the information I've shared is helpful, and feel free to reply back here if there's anything you still need clarification on or assistance with.