Overriding adb package body from a with-ed .gpr project - build-process

Question

I'm using GNAT gprbuild.
I have a project file, A.gpr that withs project file B.gpr .
There is the possibility that the B.gpr project file could be a few with-ed levels deep from A.gpr
B.gpr has source files X.adb and X.ads .
A.gpr defines a main file Y.adb that withs X.ads
Its a requirement that A.gpr must with B.gpr.
By only extending or editing A.gpr I would like to use a different X.adb .
How do i do this?
The closest thing I have found so far is https://gcc.gnu.org/onlinedocs/gnat_ugn/Project-Extension.html.
However I keep hitting the error:
unit "X" cannot belong to several projects
project "A" {overriding X.adb file path}
unit "X" already belongs to project "B"
project "B" {original x.adb filepath}

Answer 1

If files (in this case different x.adb) are in different directories it is possible by use of case statement.
For example in some of my gpr files I have following lines:
type OS_Type is ("Unix", "Windows");
OS: OS_Type := external ("OS", "Unix");
case OS is
when "Unix" =>
for Source_Dirs use ("src", "src/unix");
when "Windows" =>
for Source_Dirs use ("src", "src/windows");
end case;
As you can see, depending on variable different source directories are used.
(OS defaults to "Unix", to set variable you add
-Xvariable=value to you gprbuild call [which in this example would be
-XOS=Windows]).

Answer 2

Here's an example that uses both paths and body-selection based on project variables:
project Example is
for Object_Dir use "obj";
for Exec_Dir use ".";
type Language_Type is
("Spanish", "English");
Language : Language_Type := external ("Language", "English");
for Source_Dirs use ("src", "src/" & Language);
for Main use ("testbed.adb");
type Starts_Week_Type is
("mon", "tue", "wed", "thu", "fri", "sat", "sun");
Starts_Week : Starts_Week_Type := external ("Starts_Week", "sun");
package Ide is
for Documentation_Dir use "doc";
end Ide;
package Prove is
for Switches use ("-j4");
end Prove;
package Builder is
for Default_Switches ("ada") use ("-j4", "-C", "-g");
end Builder;
package Compiler is
for Default_Switches ("ada") use
("-gnato", "-fstack-check", "-gnatE", "-gnata", "-gnat12", "-g", "-gnatf");
end Compiler;
package Binder is
for Default_Switches ("ada") use ("-shared");
end Binder;
package Linker is
for Default_Switches ("ada") use ("-g");
end Linker;
package Naming is
for Specification("Example.Types.Week") use
"Example-types-"&Starts_Week&"_week.ada";
end Naming;
end Example;
As you can see the line for Source_Dirs use ("src", "src/" & Language); makes the src and either src/English or src/Spanish the directories that will be searched/used when looking for source files.
Later, in Naming, we have a file whose name is dependent on a project variable being selected as the body of the indicated file. -- Namely the file types-XXX_week.ada where XXX is one of the three-letter day-abbreviations the Starts_Week scenario-variable may take.