Learn Hardcore Turkish: Üst üste iki toplantı yorucu oldu.

Literally, üst means top/over, and üste is the same word with the dative -e, so the reduplicated phrase üst üste is like on top (of each other). Idiomatically it means consecutively, back to back, in a row. It functions as an adverb and naturally sits right before what it modifies, here iki toplantı.

Common synonyms:

arka arkaya
peş peşe
art arda (often heard as ard arda in everyday speech)

Why isn’t there a plural suffix after the number? Why iki toplantı, not iki toplantılar?

In Turkish, numerals are followed by the singular form of the noun. So you say iki toplantı, üç kitap, beş gün. Using the plural after a numeral (iki toplantılar) is ungrammatical.

Is iki toplantı the subject here? If so, why doesn’t it take any case ending?

Yes, iki toplantı is the subject. Subjects appear in the bare (nominative) form, so no case ending is needed. The sentence is about those two specific meetings in the past; if you wanted a more generic statement about the activity, you could say Üst üste iki toplantı yapmak yorucudur.

Why use oldu instead of yorucuydu? Are both correct?

Both are correct, with a nuance:

yorucu oldu (past of olmak) often implies an outcome or evaluation after the fact: it turned out/ended up being tiring.
yorucuydu (past of the copula idi) is a more neutral description: it was tiring.

In many contexts they’re interchangeable, but oldu can sound a bit more evaluative or result-oriented.

Can I drop oldu and just say Üst üste iki toplantı yorucu?

Grammatically yes, but then it’s a general present statement: Back-to-back two meetings are tiring (in general). To refer to a specific past experience, use yorucuydu or yorucu oldu.

What’s the difference between yorucu and yorgun?

yorucu = tiring (describes something that causes fatigue): Toplantı yorucuydu.
yorgun = tired (describes a person/being): Toplantıdan sonra yorgunum.

Could I use the verb yormak instead, like “Two meetings in a row tired me”?

Yes: Üst üste iki toplantı beni yordu. Here yormak is transitive, so you normally include the object (beni/seni/bizi). The original sentence avoids specifying who got tired by using the adjective yorucu.

How flexible is the word order? Can I say İki toplantı üst üste yorucu oldu?

That’s acceptable, but the most natural is to keep üst üste immediately before the thing it modifies: Üst üste iki toplantı yorucu oldu. Placing üst üste at the very end (…yorucu oldu, üst üste) sounds awkward.

Shouldn’t the verb agree in the plural, like oldular?

With adjectival predicates, Turkish typically uses the third-person singular even with plural, non-human subjects: Toplantılar yorucu oldu/yorucuydu. Plural agreement (oldular/yorucuydular) is mainly used with human subjects for emphasis, e.g., Onlar mutlu oldular.

How do I express a general or habitual statement instead of a one-time past event?

Options include:

Üst üste iki toplantı yorucudur. (gnomic, neutral statement)
Üst üste iki toplantı yorucu olur. (typically turns out tiring)
Colloquial ongoing/habitual: Üst üste iki toplantı yorucu oluyor.

Any quick pronunciation tips for tricky sounds?

ü (in üst) is a front rounded vowel (like French u in “tu”).
ı (in toplantı) is an unrounded back vowel; think a short, neutral “uh.”
c (in yorucu) sounds like English j in “judge.”
Stress is usually on the last syllable: top-lan-TI, yo-ru-CU, ol-DU.

Can I make the subject an activity, like “attending two meetings in a row was tiring”?

Yes:

Üst üste iki toplantıya girmek yorucuydu/yorucu oldu.
Üst üste iki toplantı yapmak yorucuydu/yorucu oldu. These sound very natural and highlight the act rather than the meetings as objects.

What’s the difference between Üst üste iki toplantı oldu and the original sentence?

Üst üste iki toplantı oldu means There were two meetings in a row (existence/occurrence). The original Üst üste iki toplantı yorucu oldu evaluates them: They turned out to be tiring.

How can I intensify the statement?

Add adverbs like çok, gerçekten, oldukça, fazlasıyla:

Üst üste iki toplantı gerçekten çok yorucu oldu.
Üst üste iki toplantı fazlasıyla yorucuydu.