How to build two product.img in one build?

[Yong Ma]

https://source.android.com/devices/bootloader/product-partitions

According to this doc, we can use a separate /product partition to contain customizations makes it possible to use a single system image for multiple software SKUs.

Is possible to build two product.img in one build? 

Or we need to:

1, trigger one build

2,  make some changes with product.img, and trigger another build.

If we cannot build two product.img in one build, it's hard to build and sign OTA.