[mia and dudley]

HI
i have a 3 yr old male shih tzu and now i have a 13 wk and 2 day old yorkie pup and training is very challenging to say the least!!
mia (my yorkie) already knows to sit for her dinner and sit for a treat she also jumps up when i say up (if i'm holding a treat )....but i taught her all these things when she is away from dudley (my shih tzu)....as soon as the 2 of them are together it all goes to pot...they struggle to understand which of them i am talking to and what i'm asking of them... dudley who previously always listened and understood everything is very confused....should i continue to do mia's training with just her there or is this just adding to the confusion??? and is it just a matter of time and i'm being impatient lol
any ideas would be greatly received
best wishes
angie