Personally, I would say "no" to this question. I do not believe that both poems send the message to readers that the object of the speaker's affection shall last forever because of the verses that he is writing. Sonnet 75 definitely sends that message. The speaker is attempting to write his love's name in the sand, but the ocean waves keep wiping it out. She teases him about the futility of it, and she says that she will fade away just like her name in the sand.

"Vain man," said she, "that dost in vain assay,
A mortal thing so to immortalize;
For I myself shall like to this decay,
And eke my name be wiped out likewise."

The speaker offers a rebuttal, and he says that he shall immortalize her name through his really great poetry writing skills.

"Not so," (quod I) "let baser things devise
To die in dust, but you shall live by fame:
My verse your vertues rare shall eternize,
And in the heavens write your glorious name"

Sonnet 79 does have elements of immortality in it; however, the immortality isn't a result of the poet's skills in writing verse. The speaker admits that his lover is physically beautiful, but he says that is not what makes her truly beautiful. She is beautiful because of her intelligence and her soul. The poet then says that those things are the truly beautiful things because they will never fade away like the physical body. The woman's true beauty, her soul, is immortal.

For all the rest, how ever fair it be,

Shall turn to naught and lose that glorious hue:

But only that is permanent and free

From frail corruption, that doth flesh ensue.

That is true beauty

The two sonnets are related, and their themes are related, but they are not quite the same. Sonnet 75's theme is much as you say: the beloved will die, but will live forever in Spenser's writing, as will their love.

Sonnet 79 does say that the object of the poet's love will live on in his writing. However, it also draws a contrast between the beauty the world sees and the beauty the poet sees, claiming that the poet sees a truer, more spiritual beauty, and that this is what will last.

So, related, but not identical.