Providing they are at the same scale and so can be aligned accurately you'll just end up with a better image than if you'd only used the first night's or the second night's data on their own. You're just adding extra data which will get included.
How the shorter frames are included depends on the algorithm you're using for combining them. If you're doing a mean/average then the 5 minute subs would slightly reduce the average exposure time per sub so will slightly darken the result but the overall effect will be an improvement due to the better SNR (when talking about the times you're using). If you're adding the frames when you combine then you simply add more exposure. Adding shorter exposures to longer exposures would effectively increase the contrast more so than brighten the image as a whole because the shorter exposures are going to be brightening the brighter parts of the image more than the fainter parts of the longer exposures.
Someone with more knowledge may prove me wrong, but that's how I expect it would work, and to some extent I've seen it myself (just not accuartely measured it 'to be sure to be sure'
).
Roger.
