Elijah Prouty, son of Elisha Prouty and Elizabeth Demond (of Rutland VT) was born August 1785 in Spencer MA and married Elizabeth Bartlet in Spencer MA October 4, 1808 and then removed from that area. I suspect that this is the Elijah you are seeing in the 1810 census. From the 1850 Census records, Chester Prouty appears to have been born around 1785 also. So while they are the same age, it does not appear that they are twins or even siblings.
I think I found something on the John you are talking about in New York and ruled him out. I will have to dig through my boxes again to see what is there.
I like to track down wills whenever possible to positively identify family members of any given Prouty family. I have transcribed many of these wills and put them on line. I haven't done any in the last two years, but hope to start up again. I was able to clear up several loose ends through these wills and correct a couple of errors that Pope made in his book. Since Wills frequently contain the names of Parents, siblings and almost always all the children of the family that lived to the age of maturity, and since they are created around the time of the actual event, I find them a more reliable source than other records. I keep hoping that I will stumble onto the Will that will provide me with the missing information and can bridge the gap between the Prouty lines.