...double up lens filters?

I've just shot four experimental frames of an outdoor subject that includes sky: one with no filter, one with a medium yellow filter, one with a deep yellow filter, and one with the two yellows together. The glass is Hoya. It will be a while until I develop this roll, and it occurs to me that I should have asked the community: I've have plenty of experience with the filters singly, but what can I expect with doubling--will the two behave like an orange, or?...do I just add the two filter factors?

No, and no.

The filters will filter out the same bit of the spectrum. That is, the second filter will have as near as no effect.

But that depends on how 'dense' the colour of the filter: the denser the filter, the more it does its job well enough to leave nothing for the second filter.

And it depends on the colour itself: if the filters not only differ in how much they pass, but also in colour, there will be an additional effect. The closer the colours of the filters to be combined, the weaker the extra effect.
Mind you, 'closeness' of colour also means that it serves no purpose to combine, say, add a yellow filter to a red filter.

Of the filters you mention, the 'deeper' filter will have a longer 'cut off' wavelength than the next lighter version, with both probably having the same transmission (next to nothing to as good as everything) on either side of the cut off wavelength. So only the deeper filter will have an effect.

So have a look at the transmission curves the filter's manufacturer publishes.

hmmm...a little more to it than I thought, thanks for the post...