What is necessary to lighten the trigger pull on a Big Bore Double? Also what should the triggers be set at to prevent doubling?

Unless you are already well acquainted with double gun triggers, this is not something for the DIY guy to attempt. And it can't be explained in words either. Different DR designs have different trigger/sear architecture. It's like brain surgery; I can explain to you how to do it, but you are very likely to kill your first few patients.

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Unless you are already well acquainted with double gun triggers, this is not something for the DIY guy to attempt. And it can't be explained in words either. Different DR designs have different trigger/sear architecture. It's like brain surgery; I can explain to you how to do it, but you are very likely to kill your first few patients.

just exactly correct

Can any competent Smith do it? Are they adjustable or is it a honing job?

Double triggers are adjustable for pull weight but that adjustment is done by removing metal.

If you get it wrong? Ooops!

So something that, like ejector work on a double, is best left to someone who earns their money on such.

Here in Britain I am blessed that I am just one hour drive from the gun trade quarter of Birmingham.

Adjusting trigger pulls on most doubles involves changing the angle of the sear nose. Less angle results in lighter pulls since you have less to overcome. Going too far results in a gun that can have the tumbler slip off the sear without pulling the trigger; usually from the shock of closing the gun, dropping the gun, or from recoil(doubling).
Obviously an intercepting sear prevents this.
This isn't a job for the inexperienced.

Send a PM to h2oboy (Dirk Schimmel) here on the forum. He's very qualified to do this.

Two alternative approaches:

Set trigger - I have one on the front trigger of my 7x65r - you press it forward and then a light pull sets it off. Some of the German / Austrian built doubles may be fitted or can be fitted with a set trigger and it does give you a very light pull. But I don't use it very often to be honest. The rifle is light so it does have a healthy recoil. A light trigger is not conducive to a firm hold so....

Modify technique - unset my trigger has a pull weight of 4 to 5lbs, are certainly heavier than my other rifles. But I have remembered an re learnt the techniques I was taught with the old Lee Enfield. Breath out, breath in, slowly breath out and squeeze both the fore and trigger hand firming up the hold on the rifle as the sights become aligned. As you squeeze the rifle is pulled in firmly, controlling the recoil and the rifle fires as you concentrate on the sight picture.

Also bear in mind that doubles are designed and meant for fluid and stressful shooting - driven running boar, or something big, hairy and that will stomp you. You won't notice a firm pull under such situations, and actually last thing you want or need is a light pull in such situations.

On mine I notice the stiff pull on the target when shooting a group. But in the field I just focus on lining up the cross hair and I don't notice the pull.