I like to think of them having a TARDIS. And well I suppose the Doctor would still only have one TARDIS, just there are two Doctors now each with their own :P I don't think I could imagine the Doctor without the TARDIS and I wouldn't want to see him permanently stuck without it. Whether or not the above is authentic or not, I still consider the cut part of Ten giving Ten II a piece of coral true, it makes me happy to think that they'll be able to grow their own TARDIS. :)