I've read a lot of material on various forums. Question: when notating two beats against three beats in 3/4 time. Obvious solution is simply two dotted eighth notes. Or quarter note tied to an eighth note, then an eighth note tied to a quarter note (beamed or not as you wish). But for a real tuplet I would write in 3/4 time three quarter notes in the lower voice and two half notes in the upper voice, adding --2-- as a bracket. Some people (and Sibelius) use two quarter notes in the upper voice. But this is short of a 3/4 measure. Is it correct?
From Wikipedia, "Tuplet": "The number indicates a ratio to the next lower normal value in the prevailing meter." In your case, there is no next lower value for two quarters. However, if you continue on reading, they mention that "in compound meter, even-numbered tuplets can indicate that a note value is changed in relation to the dotted version of the next higher note value."
My opinion and schooling has taught me that you should not notate it with tuplets at all, although you see it is common enough to be mentioned by Wikipedia. Why not just notate it with two dotted quarters? It is perfectly clear.
I've seen it done the various ways that you mentioned. Clarity is the important issue here.
You could use two dotted quarters; however, if you do this, the beginning of both beats two and three are "obscured," meaning they are hidden in the middle of the dotted notes, so this is not recommended.
You could also use a quarter tied to an eighth followed by an eighth tied to a quarter. This makes the beginning of each beat clear, but is a little clunky looking.
You could use two quarters joined by a "duplet" bracket, as you mentioned. This takes time but is a perfectly acceptable practice.
My personal preference is to use a dotted quarter followed by an eighth tied to a quarter. This only obscures beat two, but beats one and three are clearly delineated, which is a fairly acceptable practice. I personally find this method easier to input in Sibelius than the duplet method, which would be my second choice, but as long as it's clear, you can use whichever suits you, or more importantly, whichever is more clear to your end user.
Yes, the basic definition of a duplet is 'two in the time of three'. A pair of quarter duplets in 3/4 is fine.
Yes, two quarters add up to LESS than a 3/4 bar, two half notes add up to MORE. So what?
Any clear notation is fine by me. But I'm unhappy with tuplets that couldn't be expressed as a simple ratio - as in 3:2 for a simple triplet or 2:3 for a duplet.
In Sibelius, you could construct a 4:3 tuplet (don't forget the 'Other' option where you type in the ratio rather than choosing a preset) then edit each pair of quarters to be a half. But it's tricky to make the number show as anything other than 4. What ratio would you type in for half-note duplets in 3/4?
An option you didn't mention: notate the parts in different time signatures. You should only do this if you're staying in these meters for a while and bar lines must be very clear. This can potentially be confusing if the musicians don't have a conductor or haven't read the score. But if it's ultimately easier for the musician to read, it can be a good solution.
As for using tuple brackets, I agree with the Sibelius preference of using quarter notes because they match the subdivision of your intended 2/4 meter.