There are many overlap between the topics for the exams AWS Certified Developer Associate and AWS Certified Architect Associate. While some people consider Developer Associate easier, others consider Architect Associate easier. While Architect Associate focuses more on decision making (which service or feature should I chose), Developer Associate focuses on APIs and features as is. While Developer Associate focuses more on DynamoDB, Architect Associate focuses on RDS more. Many other topics such as IAM, S3, EC2 and VPC are mostly common.
From a usefulness perspective in real world, most people (including me) needs all of these and hence decided to keep these two section together. But will suggest here on what topics needs to be stressed more for each exam, like DynamoDB for Developer Associate and RDS for Architect Associate. DynamoDB is in fact a very important section for Developer Associate as there are many questions from it (at least when I wrote the exam).
Please note that notes in these sections are based on Amazon Documentation at the time of writing. Some of the features and numbers might have got changed. So please do refer to the AWS FAQs before appearing for exams. If you find any mismatch here, please do share it here and help us and others. Also, if you feel adding some additional info might help, please do update that as well.
Happy Learning and Get Certified Soon.