A_A.fsproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <GenerateDocumentationFile>true</GenerateDocumentationFile>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="T1.fs" />
    <Compile Include="T2.fs" />
  </ItemGroup>

</Project>

A_T1.fs

namespace A

type T = T1

A_T2.fs

namespace A

type T = T2

B_B.fsproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Program.fs" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\A\A.fsproj" />
  </ItemGroup>

</Project>

B_Program.fs

open A

let _ = T1 // FS0039: The value or constructor 'T1' is not defined.
let _ = T2 // FS0193: internal error: Object reference not set to an instance of an object.