What would happen if you took the deadliest diseases/viruses in the world and combined them in a single medium (a solution of water or a test subject)? Would the strongest virus defeat the rest or would a new deadly virus be created?
Viruses in the same cell can recombine (although they don't always do). This can happen by crossing-over of the genomic strands or reassortment of genomic segments. Therefore, you can get a new virus out of the fist two. The new one is not necessarily deadlier than the first two. If there's no recombination, the test subject will enjoy both diseases until the deadliest one kills him.
If you were to make a solution containing different viruses you would have a solution containing different viruses. Nothing would happen, they wouldn't "fight", they would just sit there. Drinking it would probably be a bad idea, but nothing would actually happen.
If you were to infect someone with different viruses then you would have given someone multiple diseases. Once again, they wouldn't fight, they would just go about their business infecting the host cells.
Now, it is possible that some sort of recombination would occur. Viruses work by inserting their genetic material (DNA or RNA depending on the virus) into the host DNA. Then, the cell's own machinery would start making copies of the virus. If a cell were infected by multiple viruses, it is possible that both would insert their genetic material into the host genome. If that were to happen, the likeliest outcome would be that the cell would simply start making copies of each of the viruses separately. It is extremely unlikely that any recombination would occur, but it is possible. In that case, assuming the viruses were similar enough, you might end up with a new virus, but there's no way of knowing if that would be more pathogenic, less pathogenic, as pathogenic as the original. Most likely it wouldn't even be viable and the combined genetic material of the two viruses would simply be inactive.
In short, viruses don't work the way you seem to think they do. They don't fight each other. They don't have anything resembling a mind. They aren't even alive by some definitions. It is not wrong to think of viruses as chemicals instead of as living things. They are incredibly simple and act following simple chemical rules. So yes, the likeliest outcome would be nothing interesting.
See viruses are made to destroy the host, not other viruses, so putting different types together really wouldn't have much an effect would it? as Manuella answer says, the host cell will simply suffer (or enjoy as he says) with both at once. Viruses don't mate so I sincerely doubt there would be a "combination". What some [evil] scientists are doing is that they take bacterium or viruses and they genetically modify them, and though this is a seriously tedious process, a bio-weapon could be created. Google bio-weapons and how they are made by governments.
Also, this would probably cost you a TON of money to do such a dangerous and complex project, and I doubt that anyone sensible would sponsor you.
Poison + poison = poison. They aren't integers to give you a result of 2poison. Viruses are a gatewat between living and non living organisms so they would barely mate and produce a more deadly virus. You might have been a fan of Umbrella Corporation (the company working as bio-weaponary making company which tests virus and takes over world) but in real life they don't work as we think they should.
Some virus would reverse the effect of the bad virus too. Thus as other answers has told you that being evil would cost you much more money than you think.
Bacterias are living viruses are both, so some would double up the disease whereas some might lessen down the effect of other virus.