The issue is occurring because the upgrade validation code is still being invoked, even though the serial number page is being suppressed when the alternate version (eg 64 or 32-bit) is being installed.
The issue occurs with both the Electronic and DVD products, installed using an Upgrade serial number, without a valid upgrade product installed.
For users who do not have X4 or X5, but are eligible to upgrade during a promotional period, they will need to download and install the X5 trial, using a valid retail or upgrade serial number (which should be provided to them at the time of purchase), before installing the second (alternate) X6 version.
The workaround is to have the upgrade product installed (eg X5 or X4). This will allow the second 'version' (eg 32-bit or 64-bit) to validate the upgrade.
Note: This issue has been fixed in CorelDRAW X6.1 update.