If both modules are installed the bios "should" use the
loosest, more conservative timings present, which would be
the CAS3. That doesn't necessarily mean it's "same thing"
though as running a CAS2.5 module at CAS3 will give a larger
margin for stability, and the more modules you have
installed the larger the margin (per module) would need be
in order to have stability while using the multiple modules.