How can I configure the same pool of instances for multiple ALBs, using target and autoscaling groups?

My SaaS hosts multiple domains. Each application load balancer can host up to 20 domains. Therefore I have to create multiple load balancers for my application (1 for each 20 domains).

What I want is a single pool of EC2 instances available to autoscale for traffic coming from such multiple ALBs.

Here is what I did to avoid complexities of setting up an NLB (and additional cost of NLBs).

ALB-1 -> Autoscaling Group-1 -> Target Group-1 -> Instance-1

ALB-2 -> Autoscaling Group-2 -> Target Group-2 -> Instance-1

E.g. both target groups have the same instance, but serving different load balancers and autoscalers.

The reason I do this is, why should I use seperate pools of instances that would be idle, sharing and autoscaling from one pool is more efficient.

Would this design work? What type of issues would I run into?

Edit: Problem is solved by using a single ASG for both target groups as follows:

Autoscaling Group-1 -> Target Group-1 and Target Group-2

ALB-1 -> Target Group-1 -> Instance-1

ALB-2 -> Target Group-2 -> Instance-1

Here is a key follow up question:

Are there implications if I place the same instance "Instance-1" into 2 target groups associated with 2 ALBs?

The primary reason I do this is, the first ALB/ASG/Target Group/Instance is configured by Elastic Beanstalk. I target the same instances from other ALBs, because if I manually add a separate instance to the 2nd target group the instances will not have the application backend stack that gets auto-installed by Elastic Beanstalk.

Also, at what point would the network traffic saturate and I need to add an NLB in this design?

Thanks!