From a fan perspective: it's not inherently a terrible idea as long as there is plenty of space between when they hit stops OR they are literally right on top of each other.
Going to see Dreamcatcher last year cost me just under $500. Not just money-wise, but PTO scheduling wise too, I would not be able to swing that again two weeks later. (Incidentally, this is why I did not see (G)I-DLE last year, because they came near-ish my state less than a month after Dreamcatcher.)
But if, say, Key came to my region of the country in January and then Taemin came in late March, I'd have time to save up in between visits. Or if Key came Saturday, January 7 and then Taemin came Sunday, January 8, I'd be able to combine my costs and it'd be cheaper to extend a stay than it would be to do the whole thing twice.