I just put a suggestion in factorio forums about linked belt which would permit UPS-friendly transfer between surfaces with belts. I think this is a kind of thing some mods like surfaces would really benefit.
https://forums.factorio.com/viewtopic.php?f=6&t=93017

That could be useful, but I'm not sure if the developers will bother implementing this. Right now you can get a similar result by using UPS-friendly loaders (based on inserters) combined with special chests that send their items after a set amount of time, so not every tick (at least that's how I've done it in my still unfinished reimplementation of this mod). That way it doesn't really affect UPS that much I think. But linked belts would be a more elegant solution.

I've tested the functionality, linked-belt are now a thing in factorio 1.1.6

place two linked-belt (they have a speed equivalent to a basic-belt)
set input_belt and output_belt on the two linked belts
do output_belt.linked_belt_type="output" on output belt
do input_belt.connect_linked_belts(output_belt)

items can now pass through linked-belt

Oh that's really cool! I'd love to implement this in my own upcoming implementation of this mod (https://gitlab.com/Danacus/factorio-surfaces), but I haven't released those mods due to the lack graphics, motivation and most importantly time. If you'd like to play around with this new functionality you can add it to either the "old" version of the mod or add it to the maybe upcoming mod I have been working on some time ago. If I can find some time, I'll would like play around with it myself too, because this is pretty cool. What will they implement next, trains across surfaces? :)

Don't talk about train across surfaces, it would be a nightmare to make it working mainly because trains are moving linked-objects bigger that the rails itself and colision management could be a real nightmare ...

linked-electric-pole and linked-pipe are in discussion because the implementation cost is small with a really big return value (if they connect like there is on the same network, it would permit circuitry across surfaces and make some wireless circuit even easier to make working)

I know trains across surfaces is a bad idea and is never going to happen. The original author of this mod actually suggested this a long time ago, but it was never (and most likely never will be) implemented in the game.

Linked power poles and pipes would be nice though.

This is actually really nice, I've implemented in the new mod. Thanks for suggesting this (both to the Factorio devs and to me)! Apparently there are also linked chests, that could be useful too.

I've made such mod: Linked Belts

Cool! It looks like your implementation is a bit more flexible than mine. I immediately place the second underground belt on the other surface instead of letting the player link them manually.