Evenly split 2 resources down one side of the belt

[Xarovin]

I'd like to get a perfect 3:1 item ratio on a single belt, 1 side needs to be all of one item, and the other side needs to be a 50% split.

For the sake of explaining, I'm doing it on EC currently so using copper cable + iron, but it's not going to be limited to that.

Input: 4 almost full blue belts of cable, 1 full blue belt of iron
Output: 4 red belts, one side of each one is CCCCCCCCC, other side is CICICICICICI

How would I design that with combinators? I've been trying for a while but not figured anything out yet.
The lane of only cable doesn't need to be in sync, but I need the exact 50/50 split on the other side or it'll not queue up properly at the end.

[mrvn]

First use side loading to get a belt half filled with copper cable and a belt half filled with iron on the same side. Connect that to a splitter and you get a half belt with 50:50 split of copper cable and iron. But this will break down when one of the input belts has gaps. So add some circuits to stop the belt after the splitter if the double sided belts before the whole thing aren't >5 each.

Last you simply side feed copper cables on the other side of the output belt.

[Xarovin]

Thanks, that works great, much appreciated

Edit: Unfortunately it's super slow, and the ratio breaks if adding together multiple ones

[Aeternus]

No combinators needed, just compression.
Put 2 splitters facing eachother, one field space inbetween. Inbetween them 2 belts each facing outward. Feed compressed iron on one side, compressed wires on the other. You end up with 100% iron on one side of the belt, 100% wires on the other side.
Merge THAT belt with another full compressed wire belt in a splitter and you got a 100% copper on one side, 50/50 iron/wire interlaced on the other side.

Might want to set up garbage collection at the far side to ensure you don't get resources backing up. But tbh, for EC, why not simply do 50/50 iron and copper, then put the wire-producing plant next to the EC one? When you use production mods, the ratio is almost 1:1 for those (1 copper -> 2.8 wire, 3 needed... close enough)

[Xarovin]

If I could somehow mix compression with not needing garbage collection at the end, that would work. However, I'm going for a kind of modular design, so in this case, I'll have separate iron and cable modules feeding into a central part for circuits, and all of that together will make the EC module. I just don't really have any space to do anything at the end of the belt, so I need the "error checking" done beforehand, but somehow to keep the throughput of the belts at maximum.

I attempted this before and I think I went a fairly complicated compression route (I think it'd be prone to breaking if one resource ran low), but this gives you an idea of the end result: https://i.imgur.com/8AGKFgG.jpg (actually I just loaded it, turns out I put on a little collection at the end for too many cables, but it's pretty basic)

I think some way to do with pulse detection could work while keeping it fast, just can't figure how.

Edit: Revisited mrvn's suggestion after realising I actually needed 8 belts and got it sorted, thanks for the assistance

[Hedning1390]

Also if you need an odd number a very easy thing is to mix in a filler which a splitter just before the final splitter is filtering back in a loop (since you are going from many belts to few this won't cause gaps in the final belt). I use this for science since there are 7 packs.

[Xarovin]

Yeah I only just realised splitters have inbuilt filters haha. I've finally got it all up and running, the overflow part was a little hard as I wanted to keep it as a perfect rectangle, so barely had any space to play with

Here's the bit that splits a 4+1 input into 8x 3:1 outputs, appears to work perfectly even if resources are lacking - https://i.imgur.com/ALZq41k.jpg