Afaik it's pretty common to call twinks "halfs", conjoined or not. I'm a twin, and I've been asked about "my other half" my whole life. Same thing with couples, or any two people who are perceived as belonging together for some reason.
I get that old reference, but these days and much more broadly, twink is for a term for a gay man with a certain body type, or other physical characteristics.
So you might get some odd looks using that in public.
But this is a different situation. Would you tell an amputee to break a leg? Or ask a blind person if they "see what I'm saying". I'm sure you could see how calling a single conjoined twin a "half" could be offensive.
Colloquials like that are a great way to make communication more colorful and informal, but sometimes you need to be a bit more sensitive with what you say.
yes I'd say both of these things to said groups of people at least once, if told not to because they felt it was offensive I'd obviously stop, but I believe that purposely trying to santise your own language because you assume they'll get upset is infantalising.
As a trans person I'm often on the receiving end of such treatment, people are afraid to ask me questions I'd be happy to answer because they assume they'd upset me by a mere mention of my biology.
Walking on eggshells around minorities is, though well intended, often infantalising, as if us poor weak tortured souls couldn't handle normal speech.
Talk to everyone normally and equally, unless you've been informed of ways to accommodate them better, then do it out of respect and kindness, not an assumption.
Afaik most of them hate being treated differently like that, and others tiptoeing around them in fear of saying the wrong thing. If I know someone is sensitive about it or tells me that they don't like a certain phrase then sure. But imo a good general rule is to treat them like the regular people they are, not delicate flowers not to be disturbed.
I don't speak for my clone, let alone all twins. But I don't care. I rode a bike down the side of a highway with no helmet as the logging trucks passed us by: this label will not hurt me.