the notion that someone can lose their first match and battle their way out of the loser's bracket into the finals is a very compelling one and probably creates a more competitive atmosphere. however double elim tourneys are longer than single elim and kind of a pain to keep track of, especially if you have to keep an audience aware of that information

casting is a necessity these days, even if you're not great at it. if you can, have two people casting at the same time and try to stick to the standard "play-by-play" guy and "color commentary" guy if you can. i've seen countless competitive streams marred by both casters trying to be one or both roles. if you have multiple casters that can rotate in and out, even better. no sane person will go into a small stream expecting seth killian-level commentary so just have fun with it and keep the audience engaged and aware. cover the broad strokes (overall strategy, specific matchup advantages, the importance of the match, player strengths, etc.) and don't worry about subtle technical stuff like which attacks are safe on block. assume the audience has at best a perfunctory knowledge of these games and take it from there

you could probably go to the srk forums to find some info on the logistics of setting up the tournament and maybe some advice on the technical side as well. i don't think anyone watching will be expecting AAA-quality presentation but the ability to cut to a bracket or some stats every now and then will go a long way towards making the stream look and feel professional