Although according to page 1-7 of the English version of the user manual, when you install Dual VGA, both should run at x8 speed, but checking page 1-11, M.2_1 shares bandwidth with PCIEX16(G5 )_2. When M.2_1 is enabled, PCIEX16 (G5)_2 will be disabled.
Therefore, it is recommended that you refer to the user manual via the link below to first confirm whether your two SSDs occupy M.2_1 instead of M.2_2 or M.2_3.https://rog.asus.com/motherboards/rog-maximus/rog-maximus-z690-extreme-model/helpdesk_manual/
Meanwhile, could you please provide your current BIOS version and the BIOS settings you have adjusted?
Please try defult the BIOS setting to see if the poblem able to solve as well.