I don’t fully remember if I have two lines. If I did, the second would go run to a T near the filter. I’ve seen it and done it both ways. More often than not, it is open. From a filtered air perspective, it is not needed, since there is no flow through the gauge. There might be a very slight difference in accuracy between the two….especially if the filter is allowed to get dirty or fail open. That said, I don’t believe it is a practical difference that would be noticed between annual inspections of the filter……only gross negligence of it…and then there are other practical problems….like dirt in gyros. Degradation of the vacuum pump and atmospherics make it too variable to note such a trend to begin with.