Aww. I'm thinking that if the older mouse actually is pregnant, it would have to have been visited by a wild male house mouse. However, if the mouse looks fluffed up, it could indicate other problems. I think enlarged kidneys can make mice look kind of pregnant. Is there a chance you could bring the mousie indoors and put her on some soft tissues for a night so that you can observe her pees and poos? If her pee is clear, this is often a very indicative sign of kidney failure (it means the kidney is not cleaning the stuff out - so you have low urine gravity). This can also cause a mouse (well, any mammal, for that matter) to become bloated and fluid overloaded. You know, like when people go on dialysis to clean and remove excess fluid. 
Have you looked at the mouse's nipples? If she is pregnant and somewhat close to term, I believe the nipples will be enlarged. Is your mousie eating and drinking more?
Edit -- Whoops, I seem to have missed that you said both mice appear to be pregnant. I would take both of them indoors and observe them - they might actually both be sick instead of pregnant. Mice tend to have dull, 'fluffy' fur when they get sick, especially with respiratory issues (from what I have seen).
Have you looked at the mouse's nipples? If she is pregnant and somewhat close to term, I believe the nipples will be enlarged. Is your mousie eating and drinking more?
Edit -- Whoops, I seem to have missed that you said both mice appear to be pregnant.
I would take both of them indoors and observe them - they might actually both be sick instead of pregnant. Mice tend to have dull, 'fluffy' fur when they get sick, especially with respiratory issues (from what I have seen).
