/FS is required for parallel access to a separate .pdb file

src/dds/toolchain/from_json.cpp

             if (debug_bool == true || debug_str == "embedded") {
                 return {"/Z7"};
             } else if (debug_str == "split") {
-                return {"/Zi"};
+                return {"/Zi", "/FS"};
             } else {
                 // Do not generate any debug infro
                 return {};

src/dds/toolchain/from_json.test.cpp

 
     check_tc_compile(
         "{compiler_id: 'msvc', debug: 'split'}",
-        "cl.exe /MTd /Zi /EHsc /nologo /permissive- /showIncludes /c foo.cpp /Fofoo.o",
-        "cl.exe /MTd /Zi /EHsc /nologo /permissive- /W4 /showIncludes /c foo.cpp /Fofoo.o",
+        "cl.exe /MTd /Zi /FS /EHsc /nologo /permissive- /showIncludes /c foo.cpp /Fofoo.o",
+        "cl.exe /MTd /Zi /FS /EHsc /nologo /permissive- /W4 /showIncludes /c foo.cpp /Fofoo.o",
         "lib /nologo /OUT:stuff.a foo.o bar.o",
-        "cl.exe /nologo /EHsc foo.o bar.a /Femeow.exe /MTd /Zi");
+        "cl.exe /nologo /EHsc foo.o bar.a /Femeow.exe /MTd /Zi /FS");
 
     check_tc_compile(
         "{compiler_id: 'msvc', flags: '-DFOO'}",