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No, you can't do that. Since both subdomains are served by the same content folder Apache will read the .htaccess file and apply it in both subdomains.

FWIW trying to serve static content from a cookieless subdomain is largely a waste of time unless your clients are on very high latency and slow connections. Even someone on an airplane (slow, spotty, satellite Internet) won't really benefit from it.

Your bottleneck is not in the 10 msec you'll save (at best!) with a cookieless domain. DNS resolution will take longer than that, try hosting your DNS on dyn.com or Amazon Route 53 to see a real difference. The Apache MPM you're using -or whether you're using NginX in front or instead of Apache- makes more of a difference. Static content being delivered from a server that's two continents and an ocean away from your client makes a massive difference (about 200 msec from Cyprus to California and then back), use a CDN to mitigate that. Do you have a complex template by a reputable template provider? It's adding one to two orders of magnitude more page load time than what you'll save with a cookieless domain. Use a lean, handcoded template without a template framework. Even the RegEx you're using to convert the static content links to point to the cookieless domain take more time than what you earned by not sending cookies when requesting static content. Look at more pragmatic approaches to increasing your site's speed.