Tony has given you some excellent advice!
If I could add one thing with regard to the antibiotics it would be that not only should all of the antibiotics have been finished at the time they were prescribed for the dog, not all antibiotics are able to help all conditions. If the antibiotic is formulated to kill gram negative bacteria and the dog is ill with a gram positive bacteria, the antibiotic will be ineffective. Ditto if the antibiotic is supposed to kill gram positive bacteria and the dog is sick with a gram negative bacteria. And if the dog has a virus and not a bacteria, the antibiotic will not help ANYTHING and, in fact, could actually serve to throw off the balance between good and bad gut flora and fauna leading to serious problems.
Tony also mentioned an obstruction. While this is not likely since they are BOTH vomiting and dry heaving, it also is not impossible for them both to have an actual blockage and not just a bug of some sort. I personally know people who have 3 bichons and all three of them ended up being hospitalized for bladder stones within a two week period, so while it's not terribly common, it's also not outside the realm of possibility for them both to have some sort of blockage at the same time. The point is to never use a home remedy unless it is either something VERY minor that you are treating or a stop-gap measure until the vet can see you.
Please post back and let us know how your doxies are doing after they see the vet.
Ghilly
Ok. First of all, leave them without food for 24hrs. This is quite hard for owners to do, but be firm and just stick to it. Make sure they always have fresh water available. This break from food should give their stomaches time to remedy themselves, particularly if they have eaten something that has increased the bacteria or acid levels in the gut. On day 2, give them small amounts of food. but give them these small amounts more often, say every 4 hrs. Cooked chicken and boiled white rice is fine. Never feed dogs brown rice, as it can be difficult to digest.
Second, you made a mistake with the antibiotics. NEVER give an animal antibiotics left over from a previous condition. In fact, you should never have antibiotics left over, because all antibiotics are given as a course - and the course should always be given in full and as directed, even after the dog has recovered from whatever was ailing him.
Finally, if your mother's dogs do not seem 100 per cent better by day 2, you must take them to see a vet, as it could be something more serious than a simple intestinal upset, such as an obstruction. Tony