> hows the mechanism when you have two different dimms installed?
> Is the dimms clock speed gonna drop to the level of the other one?

The Marvell northbridge always runs the entire RAM at PC2100 speed (133 MHz clock rate, 266 MHz data rate). Faster modules are thus underclocked. According to its datasheet, the MV64361 itself could run up to 183 MHz clock rate (366 MHz data rate), but bplan/Genesi configured it to 27% lower speed.

> How about timing?

I guess that the board will use the timing settings for the module with the highest latency for all modules.

> An extensive memtest was without any errors.

I recommend running an old Linux LiveCD for testing. Experience shows that even if the extensive memtest on MorphOS shows no error, there can occur "random" errors in more stressful situations like simply booting up an ancient Linux distribution.