This contains spoilers from 1968 & 1969 storylines:
Though we heard Magda say in 1897 that the curse would be on all male descendants, the writers did a bit of retconning when they had Julia explaining to Chris that the curse was passed to the oldest son of each generation (#898), which would be Chris. It was never mentioned on the show that they they were twins; only that their resemblance caused their cousin Joe to have difficulty in telling "us brothers" apart (#658). And actually, the show provides evidence that they were born at different times. This is from the topic "Tom and Chris: Twins or Not":
In ep #564, Joe is visiting Tom's grave, and the headstone reads:
Thomas Jennings 1944 - 1968
In ep #676 which taped/aired in January, 1969, Chris tells Barnabas that his first transformation occurred right after his 21st birthday, 7 years before.
These events indicate that Chris is older than Tom by a few years.