Home Networking upgrade for two houses

I'm currently under a home networking redo to incorporate a second household into the mix.
they already have two aps in their house and i have 3 main aps serving three floors and two aps for IoT devices in my house.
Can you explain what problem you are trying to solve.

Are you planning a dual WAN setup ?
 
My friend got it for combining Airtel and Jio with 300Mbps Plan but it didn't work instead the speed was dropping under 200Mbps. But when he tried downgrading the both ISP Plan to 100Mbps then it was combining it properly
I tried using it for 200+250 plan, I get 400+ on speedtest consistently.
200 is its vpn throughput max.