Yes you can. But only if the codes are of the same type. That is, you cannot enter, for example, a Ruuvi Cloud Basic code and a Ruuvi Cloud Pro code.
It is safe to enter, for example, two 6-month Ruuvi Cloud Pro codes and that would give you a total of 12 months of Ruuvi Cloud Pro use.
Note that you cannot have totally longer than 30 months of plans activated in your Ruuvi account. Meaning, you cannot activate a 24-month plan if expiration date of your old plan is not within the next 6 months.