Yes, you may attach multiple VCS roots to a single build configuration; TeamCity will checkout all of them at the start of build. You may want to set up different subfolders for checkout to avoid having data mixed up - please refer to https://www.jetbrains.com/help/teamcity/configuring-vcs-settings.html#Configure+Checkout+Rules to find out how to do that with checkout rules functionality.
Yes, you may attach multiple VCS roots to a single build configuration; TeamCity will checkout all of them at the start of build. You may want to set up different subfolders for checkout to avoid having data mixed up - please refer to https://www.jetbrains.com/help/teamcity/configuring-vcs-settings.html#Configure+Checkout+Rules to find out how to do that with checkout rules functionality.