Omg I know exactly what you mean, some of my friends actually wash them separately at the sink because they always pick up on the forehead oil I guess heh.
To keep them together you just gotta constantly comb them down.
If you cut your bangs to start like further down your head.i mean towards ure back rather than have the diviver closer to your forehead, they will be thicker and won't have any gaps, I know as I have tried both types. (what I mean is like how here see how hers starts almost at the top of her head>
link in comparison to this >
link hers start like at the forehead and will more likely have gaps
as for products, gels will make it looks more greasy, while mousse will make it stiff so that is good to make it stay in place. so either go for the mousse or a spray.