diff --git a/arm-software/embedded/arm-multilib/CMakeLists.txt b/arm-software/embedded/arm-multilib/CMakeLists.txt
index 2fa0cbb22845..2617b2c582f5 100644
--- a/arm-software/embedded/arm-multilib/CMakeLists.txt
+++ b/arm-software/embedded/arm-multilib/CMakeLists.txt
@@ -346,12 +346,12 @@ foreach(lib_idx RANGE ${lib_count_dec})
             foreach(flag ${multilib_flags_list})
                 string(APPEND multilib_yaml_content "  - ${flag}\n")
             endforeach()
-            if(ENABLE_MULTILIB_HEADER_OPTIMISATION)
+            if(ENABLE_MULTILIB_HEADER_OPTIMISATION AND variant_group STREQUAL "base_libs")
                 string(APPEND multilib_yaml_content "  IncludeDirs:\n")
                 string(APPEND multilib_yaml_content "  - ${parent_dir_name}/${variant}/include\n")
                 string(APPEND multilib_yaml_content "  - ${parent_dir_name}/include\n")
             endif()
-            string(APPEND multilib_yaml_content "  Group: stdlibs\n")
+            string(APPEND multilib_yaml_content "  Group: ${variant_group}\n")
         else()
             # In place of a json, an error message is expected.
             string(JSON variant_error_msg GET ${lib_def} "error")
@@ -362,7 +362,7 @@ foreach(lib_idx RANGE ${lib_count_dec})
             foreach(flag ${multilib_flags_list})
                 string(APPEND multilib_yaml_content "  - ${flag}\n")
             endforeach()
-            string(APPEND multilib_yaml_content "  Group: stdlibs\n")
+            string(APPEND multilib_yaml_content "  Group: base_libs\n")
         endif()
     endif()
 
diff --git a/arm-software/embedded/arm-multilib/multilib.yaml.in b/arm-software/embedded/arm-multilib/multilib.yaml.in
index 1fcc4e795d1a..e06aabba960e 100644
--- a/arm-software/embedded/arm-multilib/multilib.yaml.in
+++ b/arm-software/embedded/arm-multilib/multilib.yaml.in
@@ -20,8 +20,10 @@
 
 MultilibVersion: '1.0'
 
-# Make an exclusive group for library variants, to make sure we don't
-# accidentally include two or more variants at once.
+# Make the main library selection exclusive, to make sure we don't
+# accidentally include two or more base variants at once. But allow
+# specialised overlay layers such as string/startup deltas to be
+# selected in parallel with the base layer.
 #
 # Even if they all match the command-line options, putting two sets of
 # include directories on the include path can cause build failure,
@@ -32,7 +34,11 @@ MultilibVersion: '1.0'
 # nothing will ever include the _libc_ stdio.h, which was what the
 # #include_next was really looking for.
 Groups:
-- Name: stdlibs
+- Name: base_libs
+  Type: Exclusive
+- Name: string_libs
+  Type: Exclusive
+- Name: startup_libs
   Type: Exclusive
 
 # The list of library variants is substituted in by CMakeLists.txt, so
@@ -49,7 +55,7 @@ Variants:
 - Error: Arm Toolchain for Embedded does not provide RWPI library variants
   Flags:
   - -frwpi
-  Group: stdlibs
+  Group: base_libs
 
 Mappings: