I recommend to save slug inside article document. App can generate slug from title using Underscore.String.slugify function (https://github.com/epeli/underscore.string) when user add/edit article.
Route with multiple params are valid:
this.route('showArticle', {
path: '/:siteName/:articleSlug'),
...
data: function(){
viewData = {
title: Articles.findOne({slug: this.params.articleSlug}).title,
site: Sites.findOne({name: this.params.siteName})
}
return viewData;
}
});