What is the formula to add two of the lowests numbers in a group of cells>

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The SUM in your solution is obsolete since it sums a single value, which already is the sum of two other values (+ already means: sum the values left and right to the operator)

=SUM(SMALL(B1:B4,1),SMALL(B1:B4,2)) -- or --
=SMALL(B1:B4,1) + SMALL(B1:B4,2)

Hi

If the number of cells to be added increases it may become interesting to use:

=SUMPRODUCT(SMALL($A$1:$A$10;ROW(A1:A2)))

Regards