Can I combine two guarantors?

Absolutely. If the income of a single guarantor does not amount to 3.5 times the amount of the rent including all tax and expenses, you can combine two guarantors, each with income that is equal to 2.5 times the amount of monthly rent including all tax and expenses. 
Use the guarantee simulator on the search results and residence pages to find out the maximum rent you qualify for, depending on the guarantees you can provide.