How to keep separate materials of merged Geometry - three.js

Question

I managed to merge 2 pieces of geometry, namely plane and text. But the two materials must be separated at the end. How to do this ?
const globalMaterial = new THREE.MeshBasicMaterial({ color: 0xff0000 });
const texture = THREE.ImageUtils.loadTexture(
"http://127.0.0.1:8080/pin_png/img_gps_dasloop_online.png"
);
texture.minFilter = THREE.LinearFilter;
const workerMaterial = new THREE.MeshBasicMaterial({
map: texture,
side: THREE.DoubleSide,
});
const planeGeometry = new THREE.PlaneGeometry(1, 1);
const textGeometry = new THREE.TextGeometry(`Id : ${dbId}`, {
font: "monaco",
size: 1,
height: 0,
curveSegments: 3,
});
textGeometry.computeBoundingBox();
const workerIdMesh = new THREE.Mesh(textGeometry, globalMaterial);
const workerMesh = new THREE.Mesh(planeGeometry, workerMaterial);
// Merging
modelGeometry.merge(workerMesh.geometry, workerMesh.matrix);
modelGeometry.merge(workerIdMesh.geometry, workerIdMesh.matrix);
modelGeometry.computeVertexNormals();
const workerBufferGeometry = new THREE.BufferGeometry().fromGeometry(
modelGeometry
);
this.humanModel = new THREE.Mesh(workerBufferGeometry, workerMaterial);