Search Terms vs Tags

I have a search form on my page and need it to accept multiple keywords/terms.

If I use this url: /?tag=sport,cars&cat=4,

It shows me all the posts tagged ‘sport’ or ‘cars’ in category id 4.

However, if I use this: /?s=sport,cars&cat=4,

I get no results? It seems, the ‘tag’ url accepts multiple keywords but the ‘s’ url doesn’t.

If I only use one keyword on the ‘s’ url, it works…

Does anyone know how to fix this?