The way Twelvey speaks... makes you feel they have travelled together. Just the two of them. After Ten. But it didn't really work out for them so they agreed they're better off as friends-with-occasional-benefits instead of long-term-affair.
I mean, all the time gaps in Eleventy's last two seasons, and all Twelve's seasons for that matter. He admitted checking on Ashildr. Means he certainly does others too (Jack's OBVIOUSLY ON HIS MIND THROUGHOUT THESE TWO EPS) RIGHT? It's not like him to just starts caring about a girl he accidentally changed the life of above others. Especially not Twelve.
I mean, despite what he said, if anything the immortal ones anchors him when he's on his own for too long and is starting to lose his senses, until they reach the point of can't-stand-each-other, like canon Doctor/River does if you like, or even Missy for that matter.
I just have so many Doctor/Jack feels at the moment. To me they are practically canon now. CANON.
Fact engraved not even Moff himself can convince me otherwise. (Unless he makes them long-term-love-affair)
End of emotion wordsmash.
- So, about Jack...