Translation:Is biology or chemistry more difficult?
I agree... I'll just expand on that a bit...
"veya" may offer only two choices as well, but other options exist outside of what you're mentioning.
Geliyor mu yoksa gelmiyor mu? = Is she/he coming or not?
This is all or nothing, one or the other.
Otobüsle veya arabala geliyor? = Is she/he coming by bus or by car?
I'm asking about these two, but you could surprise me and answer "taxi, bicycle, on a donkey" etc.
As a side note, I have heard "yoksa" used in threats on Turkish TV: "Come here, or ELSE....!!!" = "Buraya gel, YOKSA...!!!" They may or may not tell you what the other option is ;-)
I'm sorry to disagree with you, as you seem very knowledgable about several languages, and I'm not a native speaker... but "yoksa" seems to be used as "or else" or "otherwise", even in statements...
Although it most often comes in question form with two question particles (mu, mi, mü,mı) as in the current sentence, statements can occur occasionally... I could only find one example on Duo: "Barda süt yoksa ben eve gidiyorum!"
But if Turkish TV can be trusted, I will also quote: "Cevap ver yoksa başını gövdenden ayıracağım!" = (literally) Give an answer or else I will divide your head from the body!"... Yikes!!! (source: Muhteşem Yüzyıl) I hear it in many threatening statements on this dramatic program (which is available on Netflix with subs in several languages) ;-)