the experiment there is a chance that an atom which sends out gamma - radiation
... and this (random) event triggers the release of poison which will kill the cat or cats.
I would view the contents of the (closed) box as one single object. What you measure when you open the box is one state - the state of all the contents of the box. Now in a macroscopic sense either both cats will be measured alive or measured dead - you can't open the box and get one alive and one dead. Either both were poisoned or neither were.
So the issue is that the box internally has a set of states we can measure it in (we measure when we open the box and look). Those measured state possibilities don't include the "miracle" state of one cat not being poisoned when the other is (unless, of course, you used a poison that's not certain to kill, which is another complication, but again just means you're measuring one state of the contents of the box).
The cat was before dead and alive at the same time.
My view is that the cats were not alive and dead at the same time, but that we simply didn't know (did not made a measurement to find out by opening the box).
The entire system is macroscopic and the random event does not alter this. The thought experiment does not work because at this scale the system does not show any quantum effects - it can be dealt with entirely classically.
If we e.g. did an autopsy ("made more measurements") we could determine the approximate time of death of the cats. In this sense these cats never existed in a state of both alive and dead, but in a state which we don't know at the time because we didn't check.
What you will not find is that you do an autopsy and check the oxygen levels left in the box when you opened it and discover they results contradict each other. Unsurprisingly, you will find the (macroscopic) cats were either alive or dead at some time, not both - you won't find they were e.g. undergoing postmortem effects while also using up oxygen at the same rate as when they were alive. So this (abstract) idea of the cats being alive and dead at the same time just won't fly.
Schrodinger's Cat is a bit of a nuisance in teaching QM because it's too complex while seeming simple. You are mixing complex macroscopic objects like cats with quantum theory, and this just confuses (IMO), rather than clarifies QM. I've never seen this thought experiment help anyone understand QM, but I've seen it confuse almost everyone who encounters it.
Could we ban teaching this ? It just generates confusion for pupils, IMO. I've never seen it help anyone understand QM.