Hmmm.....I'm no pipevine expert, but it sounds like they're both growing in shady spots (the north side of a house is shady), one is growing in pea gravel, which isn't good, you've probably had lots of rain, so the ground is constantly wet? Any of these things can make for yellow leaves. Pipevine needs about a half day's sun down here, so where you are, it probably needs more.
If I were you, I'd move both of them to a sunnier spot, with good drainage and fertile soil, even if you have to amend/make it yourself.
Pipevines aren't what I'd call picky, but they can't thrive in undesirable situations either.
Sherry

Thanks Sherry,
If I get a huge chuck of soil, can I move it now?

It's hard for me to say, knowing almost nothing about your situation, never having even been in Zone5, but probably so. Where you are it's probably still fairly cool, the ground is moist, and it doesn't sound like your pipevines have gotten very big, couldn't have much root to dig up.
Sherry

Thanks Sherry.

I have never grown pipevine, but I have had extensive experience with our alkaline pH soil, and lowering pH to keep the blueberry shrubs happy. You can immediately lower soil pH by adding 12 fluid ounces of regular grocery store white vinegar to 4 gallons of tap water, and applying this solution to the soil around the plant. This is not enough acid to stress the plant, but it will lower soil pH a bit, and in two weeks or so, you might see the plant green up. Plants do not recover from chlorosis immediately, it takes 2 to 4 weeks for plant metabolism to correct the condition.