converting a ventless suit into dual vents?”

I don't think it is possible my friend.

Better to keep it as it is if the fit is excellent.

Can only be done if there's excess fabric; probably not likely though.

If you're having the waist taken in a decent bit at the same time it maybe could be done. Probably not wise.

No.

Where were expecting the additional cloth required to come from?

I think the excess cloth can only be found if the jacket was going to be altered quite heavily (i.e. if the jacket is too big on him and would require major alterations).

You need to quite large inlays to make vents, you would need to be tailoring the coat down like four sizes for it to create enough fabric, and then the problem would be that tailoring a coat that much is crazy and you should just be buying a smaller size to begin with (and buying one with double-vents to begin with at the same time.

Short answer: No, not possible.

I think you should contact Manton and ask him his thoughts.

Rob