There’s lots of ways you can do this.
Option 1 – Project References (TypeScript 3.0+)
If you are using TypeScript 3.0 then look at using project references. Read more here.
Option 2 – Build script
Use something like gulp-typescript, grunt-ts, or just a batch script to copy the files over into a folder in the main project.
Alternatively, run a build event in Visual Studio that will copy the files over to the main project.
Option 3 – npm package
If you use npm, you could create a package for your other project. Then you can use your package in your main project. Specifying a local dependency is a good way to do this or by using something like sinopia, which is a private repository server. I’ve never used it, but it looks like it would work well.
Option 4 – NuGet package
You could look into creating a nuget package and then installing it locally.
Option 5 – --declaration --outDir compiler option
You can set the --outDir compiler option or the outDir property in tsconfig.json with the directory to your other project then also compile it with --declaration so that it generates the declaration files (.d.ts) too. For example: --declaration --outDir ../Test1/External.
Original Answer (Using --out)
You can do something similar in Visual Studio if you right click on your library project and click properties. In the TypeScript Build tab, check off the option to Combine JavaScript output into file and specify the location in your main project you want it to go (Ex. $(SolutionDir)/TypedApp/External/TypedLibrary.js). Then also check off Generate declaration files in order to generate a .d.ts file.
Once this is done, build your library project and then include the .js, and .d.ts in your main project. Include the .js file in your html and reference the .d.ts in your typescript files.
Each time you rebuild the library project, it will automatically update the main project with the changes.