Yes, I agree that there isn't very much information about these two young Grand Duchesses. I couldn't find anything about them except their dates of birth and death. They both died within two years of birth, so one can expect that they died from a childhood illness like the measles or chicken pox, or took exceptionally ill to the flu or something else. Remember, back then, it was more common for children to not survive childhood due to the lack of medical knowledge and other factors involved.
The dates of their deaths above are slightly off according to the reference I used. They did not die on the same day in the same year.
Romanov, Marie Alexandrovna
Born: 29 MAY 1799
Died: 8 JUL 1800
Romanov, Elizabeth Alexandrovna
Born: 15 NOV 1806
Died: 12 MAY 1808
It doesn't say which calendar was used in determining these dates.http://www3.dcs.hull.ac.uk/cgi-bin/gedlkup/n=royal?royal01296
The link is to a site which takes a long time to load... be patient!
And while looking around, I didn't find any mention of Price Adam Czartoryski being the father of Grand Duchess Maria. I did find however, that he was indeed the lover of Empress Elizabeth Alexeyevna, but not until after the deaths of the Grand Duchesses. Prince Czartoryski was also a friend of Tsar Alexander I. He wrote some memoirs titled, Memoires et correspondence avec l'empereur Alexandre I.
(Paris, 1887, -2 vols.)http://www.xenophongi.org/rushistory/rulers/aleksandr1.htm