LKW
Depends on the frame, older tougher frames have so much redundant strength built in (partly why they are so heavy) and can be bent or manipulated a fair bit. I am guessing a Duet falls into this category. The weak point is where the seatstays are brazed on to the top of the seatpost - worth bracing this bit before attempting any re-setting so that they do not pop off if the braze is weak or has deteriorated in the last 15-20 years. I think 10mm would be at the upper limit for a re-set (5mm each side).
I would not use 4x2, rather a long piece of threaded rod, washers and nuts, then gradually screw the ends in together. This allows for a gradual, iterative process. Wind in the nuts, release, measure, start over. You may need to clamp the dropouts down <135mm because the stays will spring out a fair way once the pressure is released.
Use a digital caliper (<$50 new, lots of applications in life) to check the measurements.
I remember there were threads about this on the framebuilding forum here at BF.
Before doing anything, discuss your options with the person who will install the disc brake tabs. They may be able to the whole shebang in one go and bring professional experience to the process. Or there may be TA options that you don't know about which could work with the existing OLD.
Post pictures!
W