DC Coefficient of JPEG Block
The DC coefficient, of the 2D-DCT (discrete cosine transform) of an 8 x 8 image block, represents the average value of the samples within the 8 x 8 block.

For your question, if you compute 8 different 1D-DCTs of 1 x 8 rows or 8 x 1 columns of this 8 x 8 block, then the DC coefficient of 2D-DCT of this 8 x 8 block will be the average of individual DC coefficients of 8 x 1 (or 1 x 8) 1D-DCTs.