How best to use cache with simplepie

First, sorry but I also posted this in the wrong forum, but its more appropriate here.

According to the docs, if you want to cache results for a fetch of RSS feeds with simplepie, you do this:

add_filter( ‘wp_feed_cache_transient_lifetime’ , ‘return_7200’ );
$feed = fetch_feed( $feed_url );
remove_filter( ‘wp_feed_cache_transient_lifetime’ , ‘return_7200’ );

My question is if I want to cache the results for several feed urls (by looping through an array), would I really want to add and remove the filter for each pass of the loop (wouldn’t that cause separate entries in the cache for each feed) or should I do this:

add_filter( ‘wp_feed_cache_transient_lifetime’ , ‘return_7200’ );

for ($i=…….){
$feed = fetch_feed( $feed_url );
}

remove_filter( ‘wp_feed_cache_transient_lifetime’ , ‘return_7200’ );

Frankly (and this might be obvious), I’m confused as to how this caching works since all I’ve seen are examples for a single feed, rather than multiple feeds.
Thanks.