How to merge two "ar" static libraries into one?

Even better you perform partial linking on each library and them make an archive of the two resulting object files. That way it operates like shared libraries would

You do partial linking with

gcc -r --nostdlib

so either instead of making the intermediate archive or after reextracting it, run

gcc -r --nostdlib $CFLAGS $OBJECTS_A -o $LIBNAME_A.o
gcc -r --nostdlib $CFLAGS $OBJECTS_B -o $LIBNAME_B.o

then

ar -cr $LIBNAME_JOINED.a $LIBNAME_A.o $LIBNAME_B.o

You can always do partial linking:

gcc -r -o libnew.o -Wl,--whole-archive libabc.a libxyz.a
ar cr libnew.a libnew.o

Here is a bash script to link all .a libs from a specified directory into a single library.

Usage: ./unify-static-libs.sh myFinalLib.a /path/to/input/libs

#!/bin/bash
#
# Creates one static library from several.
#
# Usage: copy all your libs to a single directory and call this script.
#
if [[ $# -ne 2 ]]; then
  echo "Usage: unify-static-libs.sh output-name path-to-libs"
  exit 2
fi
# Inputs
LIBNAME=$1
LIBSDIR=$2
# Tmp dir
OBJDIR=/tmp/unify-static-libs
mkdir -p ${OBJDIR}
# Extract .o
echo "Extracting objects to ${OBJDIR}..."
for i in ${LIBSDIR}/*.a
do
    echo $i
    ar --output $OBJDIR -x $i
done
# Link objects into a single lib
echo "Creating $LIBNAME from objects..."
ar -crs $LIBNAME $OBJDIR/*.o
# Clean up
rm -rf ${OBJDIR}
echo "Done."

ar -x libx264.a
mkdir sub && cd sub
ar -m ../libx264.a `ar -t ../libx264.a |sort|uniq|grep "\.o"`
ar -x ../libx264.a

now you have two version of "macroblock-10.o"

ar crsT libaz.a libabc.a libxyz.a

Here, you create archive of archives and then 'flatten' (thinning) result with T flag. Not sure how it'll work with same name .o files that might be contained within.

This question is specific to Linux. If you're looking for how to do this on macOS, the answer is libtool, as explained here:

libtool -static -o new.a old1.a old2.a