DO IT.
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IKEA rates horizontal surfaces and if you want assurances, each SHELF is recommended for use up to 66 lbs (there are two shelves in that unit, plus the top). But putting another Besta on top doesn't put stress on the shelves (or top) it's all going to be on the vertical parts which are all also quite solid - the same things that support the 3x1 shelf itself. Totally fine as far as weight goes.
The most important thing is making sure the pieces are joined together well and the whole new tall structure isn't tippy at all.
That extension unit, covered with doors especially, will look stellar lifting up other Besta shelves.