MB is absolutely right. Just go ahead and putt as if the other ball wasn't there.
(This is one area of the game where the laws of physics definitely do not apply!)
... however, this situation, known as the "stymie" used to be a perfectly legal part of the game of golf, wherein players could deliberately try to leave their ball in the way of their opponents path to the hole.
The earliest forms of golf in Scotland were predominantly matchplay games, where foiling your opponent was at least as important as making the lowest score yourself. Players put in a stymie would have to try and putt round, or chip over the other ball to get into the hole, and if they knocked their opponent in, then that was good luck for the opponent.
Bobby Jones was a huge fan of the stymie, believing that it added an additional element of strategy to the game. I believe it was banned in 1952 when the R&A and USGA finally established a joint set of rules.
BTW: This happened to me this morning in a Best-Ball Pairs tourney. My partner and I both ended up on the exact same line at #4 at RSG, with me a couple of feet further away. When I saw the outcome of my birdie attempt, which went right through my partner's ball, I was able to pass on, via Chat, the precise aiming line so he could sink his birdie.