aws ec2 instances in different vpc subnets access each other

I have 2 AWS EC2 instances living inside 2 different subnets of my vpc.

I would like to allow the ruby app running on the first instance (say App#1) to call the endpoints of the app (say App#2) running on the 2nd instance.

I would also like my users to directly call the endpoints of App#2 from their browser.

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Here is what I have tried (and mostly failed):

1. [Sucess!] I added the known IP addresses of my users to the inbound rules of Load Balancer Security Group of App#2 and have confirmed that they can access App#2 endpoints from their browsers.

2. [Fail!] I added the Load Balancer Security Group ID of App#1 to the inbound rules to the Load Balancer Security Group of App#2. But my logs tell me App#1 cannot access the endpoints of App#2.

3. [Fail!] I added the VPC Security Group ID of App#1 to the inbound rules of the Load Balancer Security Group of App#2 - nope, still doesn't work. (Somehow, when I launched the instance for App#1, aws automatically created 2 security groups for this instance - one for VPC and one for load balancer... I have no idea why/how this happened...)

4. [Success...Sort Of] I assigned an elastic IP for the instance running App#1 and added that to the inbound rules of the Load Balancer Security Group of App#2. This works but I would rather not use this method since I would like to elastically scale my App#1 in the future and I do not know how to automatically assign more elastic IPs for the new instances when they spin up, add them to the inbound rules, and then somehow remove them when they shut down.

I feel like there has got to be a really clean solution to this problem and I am probably missing something painfully obvious. Can someone please give me a hint?

Any help would be appreciated!