Can you expand on the bit in bold?
Do you mean you can send Pr coins to a Un client? and vice versa?
Yes, that's correct. Pr and Un/testnet are completely cross compatible.
You can't send Pr to an Un mainnet addy--they don't use the same leading bit in the address (u vs U) but from the Un testnet to the Pr mainnet and back again works perfectly!
Sorry to come off curtly--I'm not trying to dilute or take anything from Un, just give some new things to play with as Un development has moved slowly in my time away.

OK, if I understand you correctly this is NOT actually the Un testnet but rather it is a parallel chain of the Un testnet? i.e. there are two separate blockchains (Pr and Un testnet) and transactions can be sent between the two?