First, you have to have strong personalities. I reckon most of the time should be spent creating your characters - no one wants boring, bland characters. Boring characters = no fizz = no chemistry! If you are creative, any combinations of characters could work. If they're quite similar, build them together through their love of something they both find passionate together, if they're quite contrasting, you can create fire and energy like friction through playful bickering, leading to more dramatic emotions? Have fun & good luck X
Umm well have a good setting and reasoning for why you want chemistry between them. And for good chemistry give them a history and challenge there chemistry with different moods and obstacles. thats just my opinion i love writing.
