Attend to and tend to are synonymous in two senses: (1) to work for or be a servant to, and (2) to apply one’s attention to. This covers most of both phrases’ territory. Where they are not synonymous is where tend to means to have a tendency to.
Both attend and tend can be intransitive—that is, used without the to—but often the to is necessary to avoid confusion with other senses of attend and tend.
For example, the writer wisely includes to in this sentence because attend alone could cause confusion:
Those that attend to the long-term energy needs of their companies will gain an edge over those that don’t. [Forbes]
When the to can be dropped without creating confusion, attend on its own carries the same meaning—for example:
Instead, time and again and in appalling weather, he went into no-man’s-land to search for and attend the wounded. [Telegraph]
With tend to, the to more often bears removal. For instance, to is unnecessary in these cases because one sense of tend (without to) is to take care of:
Animal Control worked with the man to reduce the number to the legal limit of three, then left him to tend to his pets. [Chicago Tribune]
Her maternal urge to tend to her baby trumps the physical pain she has to endure. [Herald Sun]