You don't have to write good arguments for two characters arguing opposite points in a debate, you need to write good characters.
As someone writing a debate, it's likely that you will favor one side over the other, therefore bias to the debate will likely creep in, as it is difficult to write arguments to support a point that you don't believe in. You will feel the person who is arguing against how you feel personally doesn't have good points, because you don't personally think they are good points. The trick is then to make sure that the character truly believes in the points that they are arguing.
One good thing to do would be to practice writing out a debate about something you are 100% decided about, let's say the Earth not being flat. Then think about how someone trying to argue that the Earth is flat would attempt to convince people.
They may resort to appealing to emotion over solid facts, misrepresenting statistics, using specific theories or studies that agree with their point of view or any number of logical fallacies.
Then when writing about an issue that may actually have two valid sides of an argument, you can intersperse tactics that someone would use to argue a lost cause with actual solid reasoning that they may use. Depending on the characters debating, they may both resort to some of these tactics to some extent.
It is not about writing a good debate, it about writing how characters would attempt to debate well. Whether their points are good or believable or agreeable is irrelevant, it is about how they are argued by the characters you have written.