Your odds of a win are the same for each line you buy. So if the odds were 1 million to 1 each line would be 1 million to 1. The only way your odds would increase is if the lines were combined.
Buying more lines means you have more chances at the same odds.
Which means the total odds of your ticket winning are reduced. The odds for each line are the same, but the odds for your ticket get better, the more unique lines you play.
So a ticket with one line at 1/1m has odds of 1/1m of winning the jackpot.
A ticket with two lines at 1/1m has two chances of winning the jackpot, so 2/1m, or 1/500k.




