Hi, I'm just starting usage of growthbook and we have single mobile app but 2 different websites (separate brands, that in mobile app live together but on web have their own domains as eCommerce shops). Currently I assume having single project in GB and using user attribute "brand" is best solution but maybe somebody already went through such challange?

there are a few ways to organize this, depending on your needs. We often see multiple sites managed with attributes if the flags and experiments are roughly the same across both. If you want to usually target to one or the other, using Projects is a better way.