Loosely coupled microprocessors are more like independent processors joined via a small communication link (a high speed bus or cable). This means that the connected microprocessors have their own local memory sets. They are low in performance. Also only microprocessors that can work independently may be used.
Tightly coupled microprocessors share a common memory for the purpose of communication. One processor (slave) is dependent on the other (master). They are good at performance. The connected processors also have local memory sets for general purposes.