My 2005 daughter still plays both. She is actively sought out for guesting with select and extra teams. But she only plays the field, mid or forward, while guesting. Because she mostly plays keeper for club, has a high engine that keeps here playing outside every day no matter what(mostly basketball on her off days), it makes sense for her to guest play.
We make sure all her coaches know what is going on, and that when a conflict may arise, club always comes first. I will say there are a good many select/extra teams that are better than many of the tier 3 / bronze club teams.
I think every kids situation is different and we as parents must identify if playing with two clubs is good or bad. If it allows your kid to grow as a player, learn new positions, and have fun without them appearing tired, then go for it. I've started to notice a lot of boys and girls that still do both.