Please could somebody explain Portguese/Brazilian surnames?

[reallifeboogie]

For example, if a persons full name was Lucas Lobo dos Santos, would they go by both surnames on a day to day basis or would they go Lucas Lobo or Lucas dos Santos?

Both.

Your full name would be name and two surnames for everything.

Often just the last name is used (in Portugal, anyway). You can have up to 4 family names. Maternal names come before paternal names,