I would definitely ring the first guy first & tell him that if the work is not done within the week you are contracting someone else to finish the work & no more money will be paid to him..........it usually takes ultimatums to get that type of guy to finish....you may be pleasantly surprised what threats can do.
But if he doesnt show during the week then I would defo call someone else in.
Its a pain in the ass - I would prefer someone to tell me that they cant do the job for a couple of weeks & give me an actual date than to keep telling me they will do it tomorrow!