Formula for ohms in speaker chain
Series connection: \$ R_{total} = R_1 + R_2 \$.

Two 8 Ω speakers in series -> 16 Ω.

Parallel connection: \$ R_{total} = \frac {R_1 \cdot R_2}{R_1 + R_2} \$.

Two 8 Ω speakers in parallel -> 4 Ω.