I am glad that your vet prescribed probiotics and vitamins as a first step for your cat’s problem. There are several medications that can be administered to cats with stomach issues. However (as I discussed yesterday), if the syndrome can be treated effectively by more natural means, there is no reason to medicate.
To answer your question, I see no reason why you can’t mix the vitamin B-12 and the probiotics together into the wet food. Very high concentrations of B-12 could conceivably inactivate the benign flora in the probiotics. However, if you thoroughly mix one drop of B-12 into the wet food before you add the probiotics, I sincerely doubt any problems will occur.
I have two additional points to make. First, remember that diet often plays a role in digestive problems in cats. If your cat is not already on an easily digestible diet and you have trouble managing his problems with the regimen of B-12 and probiotics, talk to your vet about switching foods.
Second, some people might get the impression from my last two posts that I am opposed to prescribing medication for my patients. That is not the case. Medications, when prescribed appropriately, can be incredibly beneficial to animals and humans. What I object to is using medications inappropriately or unnecessarily.