Can I perform Ashtami Shradh and Navami Shradh on consecutive days?

Yes, absolutely. Many families perform Shradh on consecutive days during Pitru Paksha for different ancestors. Ashtami Shradh on October 3 and Matru Navami (Navami Shradh) on October 4 are completely independent ceremonies for different ancestors. If you have ancestors who died on both these tithis, you can arrange both through your pandit — either at the same location or at different locations if circumstances require.