A cistern can be filled by one tap in 8 hours, and by another in 4 hours.

Time taken by the first tap to fill the cistern $=8$ hours

Time taken by the second tap to fill the cistern $=4$ hours

$\therefore$ Work done by the first tap in 1 hour $=\frac{1}{8}$

Work done by the second tap in 1 hour $=\frac{1}{4}$

$\therefore$ Work done by both the taps in 1 hour $=\frac{1}{8}+\frac{1}{4}$

$=\frac{1+2}{8}=\frac{3}{8}$

Thus, both the taps together will fill the cistern in $\frac{8}{3}$ hours or $2 \frac{2}{3}$ hours.