Yes, AWS doesn’t support master-master replication on RDS but we you can take a different approach, master-master replication between RDS and External MySql Instance. And the External MySql instance might be EC2 or your own server.
So, scaling horizontally for writes is not possible. RDS does support the ability to create multiple "read only" RDS slave instances, so scaling horizontally for reads is possible. Make sure to check about Amazon Aurora Multi-Master