I've noticed two accounts that have had the same last name, same general answer, the same source, and same joining date. However, they do not seem to be sock puppets as the two answers by the accounts have no up-votes. What is the response for situations like this?
There is no rule that a person cannot have more than one account, provided they follow the rules. As long as they aren't spamming or sock-puppeting it's allowed.
Its not really encouraged though as it can be difficult to manage, but can be useful if you want to differentiate between regular you and professional speaker of X(University/museum/institute/society) you.
Just to get the terms right, if I had a second account, that in itself would be a sock puppet. Sock puppets are 100% fine as long as they don't break the rules. The most frequent violation of a sock puppet is voting for another account of the same person (AKA fraudulent voting). The other common violation is spamming. When sock puppets do engage in these malicious activities, they have broken the rules and will be prosecuted (i.e. suspension, merging, deletion, etc.).
Also important to note, the mods have powaz (shiny tools from the dev team) to locate and stop fraudulent activity of sock puppets. Judging by the info we have, I doubt they are actually sock puppets. For all we know, they could be relatives interested in similar areas of history who have invited others to SE. That's great! Always assume the best of users. When the mods do find fraudulent activity (whether by socks or non-socks), be assured they will deal with it fittingly.
See I am a sockpuppet, may I exist if I have a real purpose? for more information on socks and their limits.