Unfortunately as of writing this (Oct 18, 2017) there is no built in integration for multiple target groups per AWS ECS service. Here are a few things you can try:
If your application just serves port 80 (HTTP) & port 443 (HTTPS) then you should consider using the application load balancer and terminating SSL at the load balancer. This will allow your application to function using just port 80.
If your application serves different ports that are backed by different components, perhaps speaking different protocols then you should consider splitting the application into multiple ECS services. This has the benefit of allowing the different components to independently scale