As-Salamu alaikum brothers and sisters, i heard that Duma should be prayed two by two, that's saying the Salam in each second rakah, does that mean that (1) you have to announce (by heart) the intention and takbir l-ihram again? (2) and are witr and qiyam al layl also two by two? (3) and is saying Salam after each two rak'ats fardh? Because I prayed 12 rak'ats of duha with only saying one Salam. (4) and after you say the Salam, are you able to move to another place and finish the prayer? Jazakum Allah khayran...
Even if there might be a difference of opinion on the amount of raka'as of sunnah or nafl prayers once you know that you have to pray them two by two or as sets of two raka's it should be clear that each (set) of them starts with a takbeer and ends with a tasleem. So you start with a takbeer (al-Ihraam) and pray two raka'as and say salam. For the second set you say your takbeer pray two raka'as and say salam again and so on.
The key of prayer is purification; takbir (saying "Allah is most great") makes (all acts which break prayer) unlawful and taslim (uttering the salutation) makes (all such acts) lawful.
(Sunan abi Dawod, Sunan ibn Majah and Jami' at-Tirmidhi)
Yahya related to me from Malik that he had heard that Abdullah ibn Umar used to say, "(Voluntary) prayer in both the day and night is two at a time with a taslim after every 2 raka'ahs." (al-Muwatta')
The Messenger of Allah (ﷺ) prayed on the day of the Conquest (of Mecca) eight rak'ahs saluting after every two rak'ahs. (Sunan abi Dawod)
'A'isha, the wife of the Messenger of Allah (ﷺ), said that between the time when the Messenger of Allah (ﷺ) finished the 'Isha' prayer which is called 'Atama by the people, he used to pray eleven rak'ahs, uttering the salutation at the end of every two rak'ahs, and observing the Witr with a single one. And when the Mu'adhdhin had finished the call (for the) dawn prayer and he saw the dawn clearly and the Mu'adhdhin had come to him, he stood up and prayed two short rak'ahs. Then he lay down on his right side till the Mu'adhdhin came to him for lqama. (Sahih Muslim)
As a set has ended you may move and pray elsewhere.
As for your intention you may need to do a new intention for each of these sets. Basically the intention is made in the heart by a simple thought at the beginning of an act.