How to merge multiple tilesets into a single one?

I have seven tilesets having each AssetIDs supported by cesium ion. Each tileset includes 3D buildings covering its administrative area. For the project area, 3D building data was divided by 7 sectors for the sake of performance and safe upload to Cesium ion. However, accessing the building theme alternating the AssetID was troublesome. I want to merge the multiple tilesets into a single tileset. How can I do it?

FYI, from client side in CesiumJS, I used the array to treat the tilesets like a single theme. Here is the example;

        var tileset = null
        array = [ 265710, 265727, 265780, 265463, 265290, 265395, 265431 ];
        for (index = 0; index < array.length; index++) {
            tileset = viewer.scene.primitives.add(
                new Cesium.Cesium3DTileset({
                    url: Cesium.IonResource.fromAssetId(array[index]),
                    maximumScreenSpaceError: 16,
                    maximumMemoryUsage: 512

But it shows a performance issue for the assets in the latter part of the array (e.g. 265395, 265431). Accessing the AssetID 265431 takes very long even when the screen tries to display the corresponding area only.

Can I get some help?