I agree with you that it is strange for H-P to start developing two different versions of the same product in parallel.
My understanding is that there will be an upgrade path from 4.5 SP18 to 5.2. To my knowledge, there was never yet a date announced for 5.2. I am also assuming that there will be an upgrade path from 5.1 to 5.2.
IMHO, the best way to know the exact dates that all these versions will be released is to do divination based on sheep livers, like the Babylonians did.