I tried to install the 64 Bit python version (2.5.1) and the 32 Bit version on my computer. But it is not possible because the installer complains that this version of python is already installed.

Is it in general not possible to install both versions (in separate directories) ?

Or is it a bug ?

It's not possible, because they use the same program GUID. You can use an MSI editor to change the program GUID, then you should be able to install them simultaneously (you may have to change the UpgradeCode as well, as otherwise installing one might silently uninstall the other). Of course, both installations will stomp on each other's registry keys, so while this MSI conflict is not deliberate (I never considered that setup), it may be a good thing that it already breaks at installation time. Regards, Martin