There is no easy or good way of doing that. Tank chassis and turret could be in different files with tankname.vxl and tanknametur.vxl.

For same voxel file:
- Can import a section from a different voxel file, making it a multi-section voxel. Then use HVA Builder to align those.
- Can open the 2 voxels in 2 different Voxel Section Editor and align the canvas. Then copy layer by layer from one to another. Then redo normals.
- Can use XCC Mixer to do the same by exporting the layers into 2D images of both voxels and then combining in some image editor and then use XCC Mixer to assemble them back. Then redo normals.

I think there should be a tutorial on this somewhere. Anyway, here's a rough one.

Save your two voxels (let's call them hull.vxl and deck.vxl for this guide).
* For convenience, change the canvas size so that they are both the same dimensions.
* Align them (by using the nudge function on VXLSEIII). Use reference points to algin them so that if they were combined without any changes, they would be in the right positions.

Once you've saved them, open XCC Mixer, and browse to your working directory. Right click on each voxel, and choose "Copy as Text". This will create two text that we will be using to merge the voxels.

Create a new text file, let's call it output.txt. Open all three files, and copy the contents of hull.txt and deck.txt into output.txt, one after the other. Save output.txt.

Go back to XCC Mixer, refresh and right click output.txt, and chose "Copy as Voxel". This will create a new file, output.vxl that has the content of both the old files in one section.

Open this file in VXLSEIII. If it looks good (positioning, scale), kick off autonormals and you're good to go.

If it looks out of position, you'll need to open the original vxl to reposition one them (with respect to each other), and then redo the conversion through text.



Things to note:
* This only works well when both files are the same scale.
* XCC Mixer doesn't convert normals. You'll need to do them yourself after converting.

Just for reference, I have a tanker truck made with 2 sections (cab+trailer) and I released a dumper truck with a plow some people wanted to use as a harvester with 4 sections. As long as the sections aren't animated the game handles their rendering priority well.

Alternately resize the canvas to be the same dimensions in at least one plane, then with 2 instances of VXLSE copy and paste from one to the other where they don't overlap. Then fill in the rest manually.

Just for reference, I have a tanker truck made with 2 sections (cab+trailer) and I released a dumper truck with a plow some people wanted to use as a harvester with 4 sections. As long as the sections aren't animated the game handles their rendering priority well.

Alternately resize the canvas to be the same dimensions in at least one plane, then with 2 instances of VXLSE copy and paste from one to the other where they don't overlap. Then fill in the rest manually.

Like creating a mirror in the same section using VXLSEIII? I did this in very few voxels such as the YR Battle Fortress, so that it would have additional 2 tank treads in the top alongside the bottom.

It needs some prepreparation, but it is indeed possible.

When you convert from vxl to txt in XCC Mixer, the text files are simply a comma-separated list of pixels and their locations, but only if the pixels is actually colored. Because of this, simply pasting the contents of a second file is enough to effectively merge the two. It would have been great of XCC also converted the normals, but it is what it is.

Combining several different voxels into one by means of sections is one of the things I tried early on in my modding career, over a decade ago. IIRC, it was a farrago of several tanks to try and make a badass looking one. It failed miserably for various reasons (shadows, locations, etc), until one of the veterans (either Mig, or DC, not sure) pointed me to the method above.

EDIT: I've attached 3 files:

CDESTWO is the coastguard destroyer without a spawn, while
FALC is the harrier - both have had their canvas changed to 43x43x120. The ship was nudged forward, while the aircraft was nudged up about 10 pixels

BDEST is the other two files combined through text. The normals had to be changed back to 4 (Ra2) and autonormalled. The voxel bounds were also reset. The whole process took less than 10 minutes.