You could just add one then add the other a few hours such as 6 hours and i think it would be fine. Ive never added both of those before though.
I don't think that that is good idea, most medicines are active for around 24 hours. 6 hours is not enough, it still opens the door for possible interactions.
Very similarly, this is why you tell the doctor and pharmacist what other medications and supplements you are taking. There is a chance, albeit a small chance, that interactions can occur. Even then, the most likely interaction is that one medication will interact with another, rendering one or both of the medications ineffective. This is bad because then you aren't getting the medication you need. However, sometimes they can react together to make dangerous compounds. You just don't know. Most of the time, two medications won't interact, but you always check to make sure, and never assume that they don't.
Now, the above example is for human life, and these are just fish. (I'm hoping to NOT start a debate on human life versus fish life.) Nevertheless, there is a chance, again a small one, that the two medications could interact. If you know that they won't, then you can mix them together. But, I would never assume that they can be mixed together. Because you just don't know what will happen. Unless the package specifically says you can mix it with another medication, or unless you can find a study that shows they don't interact and both medications remain effective, I wouldn't assume that it is safe/ok to do so.