HI @Milen Denev
You can use one Loab Balancer but to use the same port (80) to different backends you need to have two differentes frontend IPs.
Load Balancer is a Layer 4 and work with TCP Ports, you can use a Apllication Gateway to address your need. Application Gateway is a Layer 7 and it can publish differents urls using the same frontend ip.
See more information on this link: https://learn.microsoft.com/en-us/azure/application-gateway/create-multiple-sites-portal