Not knowing the song, but assuming the beginning is too low, and the key changes are all upwards, which is usual, I'd start in maybe the 2nd key, and either lose a key change, or go up another time, if the duetter can cope.

Changing key for a song is, as jjmusicnotes points out, very common - I did it all the while when playing for choirs, often to get a better sound from them, instead of having them screaming at the top of their range, or grunting in their boots.

Seeing the total song may give other options. The above options may also not be good for accompanying musicians.

Since the start comes straight after some narrative (I'm guessing), the easiest would be to speak the first bit, before bursting into song, maybe on the second or third line.

Alto singing Tenor
I had an idea for a Mutants and Masterminds (2e) character based on a video game character idea of mine: a Gestalt consisting of a human male and a smaller, roughly cat-sized alien symbiote. As a Gestalt, the symbiote could provide augmentations for the human by fusing its body mass with various regions of the human's body (i.e. arms for strength or reach, legs for speed and jump height, forming wings on his back for gliding), but due to its small size, it can only provide one benefit at a time, but can take an action to switch what benefit it is providing. 

Individually though, things work differently; the human doesn't have any powers of his own (except maybe above-average physical stats as a side effect of his little friend) while the symbiote not only has small size and shapeshifting abilities, but also can bond with and control other humans to aid and protect his owner, though it cannot provide the same augmentations as when it is bonded to its owner.

I am aware there are powers available for this concept to work, I'm just wondering if the whole "human without powers + small non-human with powers" joint combo could be done within the game's rules. Is it possible, and if not, what would I need to change?
