It is easier to work with a single supplier than multiple suppliers because communication is faster and you can benefit from the perks that come with selling high numbers like discount offer and net 30 payment much faster with one supplier than when you are splitting orders across multiple suppliers.  It's allowed to work with multiple suppliers and I encourage it when you are already getting a large number of orders in your store.  Having a main supplier and an alternative supplier for urgent days, is the most recommended option.