Filtering entries by multiple categories with multiple filters
Passing the category as a comma delineated string and using craft.request.getParam('category') the second time will not grab the second variable 4rooms.  First you need to parse that string into an array. This will create an array ['forsale','4rooms']. Use the parts of that array to assign to your other variables.

{% set params = craft.request.getParam('category')|split(',') %}
{% set offerCategory = craft.categories.slug(params[0]) %}
{% set offerRooms = craft.categories.slug(params[1]) %}