If you partition a >2TB volume as an MBR volume to allow booting then it's limited to 2Tb, any space beyond that is inaccessible.As I understand, if, say a 4TB HD did exist, there's nothing stopping users making 2 equal sized partitions for 32bit OSes - or a 1TB NTFS boot partition and 3TB GPT data partition under 64bit.
The MBR/GPT choice is set at the volume level, not at the partition level and hence can't be mixed on a single disk.