Sorry to hear there's fighting going on - I know it can be quite stressful when pets who used to get on start arguing. You were absolutely right to book the terrier in to be spayed, and your vet was right to delay the op until the phantom pregnancy had time to settle. The spay operation can be dangerous if performed at this time.
Dogs originally used to run in packs, and the fertile females would usually be either pregnant or nursing most of the time, so their wombs are primed for this. If they are not mated, the womb often seems to 'assume' they are pregnant anyway, resulting in a false pregnancy. What has happened in your terrier's case is that the hormonal changes have altered her behaviour to aggression. This is quite common with false pregnancies.
I would suggest you pop down to your local pet store or vet and get a comfortable muzzle for the terrier to wear, and possibly one for the cocker. Identify when fights occur, e.g. feeding, and separate the dogs at this time. This won't have to continue forever, as in my experience the aggression settles down after spaying.
If things are getting on top of you, talk to the vet you saw about the spaying. You may have to pay for a face-to-face consultation, but most vets are quite happy to field phone calls for free if things haven't worked out yet. If this is the case, there are drugs which can hasten the end of a false pregnancy so you can get back to normal quicker.
All the best,
P.S. It's Lhasa apso, if you were wondering.
Hi there, Vet Andrew here. How are you getting on?