I had exactly the same problem you have, with almost the same colours: two blues. My solution was very similar to what Scott suggested in his answer but, (instead of overprinting one gradient on the other one) I ended up, by the suggestion of the printer, overprinting a gradient of 0 to 100% of the darker blue over an area of 100% of the lighter blue.
This is what I originally did (no overprint), and the result was awful:
This is what I ended up doing
The are two reasons why I did it this way:
As Scott suggested, the reason for overprinting is that when you have two gradients, one that goes from 0 to 100% an the other one going for 100 to 0%, there will be an area of lower density where both gradients meet, sort of mid way (depends on how fast your gradients change). There will be less ink in that area so the resulting colour in this area can look kind of washed out.
Why one gradient over a 100% flat area?
The reason for this was to avoid banding. When you have two gradient screens printed one on top of the other, if they are not perfectly registered, they can create an artifact that results in visible bands of colour instead of smooth gradients. I am sure you have seen this in low quality prints.
Word of caution: depending on the transparency of the darker ink, the colour at the bottom of the gradient, where both inks are at 100%, might not be exactly the darker PMS but a mix of both PMS inks.
This is how it ended up looking in different strata:
Metal and Plastic (including labels). These strata are less absorbent so the overprint result is a bit bolder.