Specifically I am looking at a QOTO2020:
You can see what seem to be holes in the handles for a connection pin.
On a single pole breaker I can't think of an application where you would want both independent circuits to switch off in the event of a fault on one of them. Unlike a 2 pole (USA) breaker where you would want both legs to go off in unison.
What use would be pinning the two handles together? Or are those holes not for that purpose?
You need to tie breakers together in certain scenarios.
One common scenario was 15 amp kitchen plugs in certain jurisdictions where on a single device you have severed the outlet tie and have two circuits powering the single device. When working on the outlet you want to flip the breaker and not be able to have half of the device live.
It's to make what are called "quad" breakers, where you have two twins side-by-side, creating 2 two-pole breakers out of 2 slots instead of 4 (or, as in the photo below, one 2 pole + two 1 pole). YOU cannot do this on you own, you would have to buy them AS quad breakers, but when they make the twins, they make them all the same.
if you put it next to another similar breaker you can have a 240V circuit where they abut.
That is, you can join one handle to the neighbor breakers handle to make a two pole breaker
The question isn't whether the breakers are able to accept a connecting pin. The question is whether the manufacturer makes a UL-listed handle tie for those pins. I would bet they do not.
There might be a reason to handle-tie the inner breakers of two adjacent duplex breakers; i.e. In an A1 A2 B1 B2 arrangement, tying A2 to B1. That would be for the normal uses of handle-ties, e.g. Multi-wire branch circuits and 240V-only loads.
Also keep in mind, handle ties do not assure common trip; multi-pole breakers have an internal mechanism which does that. (Which also means a multi-pole breaker is much more than two singles bolted together). The purpose of handle ties is common maintenance shutoff, needed for 240V loads (which can have each leg separately fused), MWBCs (so you don't get nailed by the other leg), and 2 circuits on 1 yoke (ditto).
When you use a handle lock device the screw goes into handle hole.
I don't think these are OSHA approved as lock-out/tag out, but are used as lock on for Fire Alarm Panel circuits. When used as lock-on the breakers still trip, but it prevents inadvertent switching off.
