Steve left Kristina to deal with the creditors.
Under a grammatical perspective, could Steve be the doer of both verbs left and deal?
If so, can anybody explain what the implications on the sentence's meaning are?
This is a semantically ambiguous sentence precisely because the grammar allows for different interpretations, depending on the intended function/meaning of "left".
Steve [went away, leaving] Kristina to deal with the creditors.
In this interpretation, Steve left and Kristina dealt.
Steve [parted from] Kristina [in order] to deal with the creditors.
In this interpretation, Steve both left and dealt, and Kristina was merely the object of the leaving. (For what it's worth, this was my first interpretation, and I had to "twist my brain around" to see the other possible interpretation.)
In the sentence you wrote, Steve could certainly be the subject of both actions; my understanding of your sentence is that Steve was with Kristina and at some point he left her in order to deal with the creditors.
If on the contrary the idea you want to convey is that Steve didn't want to deal with the creditors himself and wanted Kristina to do it, then it would be clearer if you said that he let her deal, or asked her to deal, with the creditors. The way it is written, your sentence can be interpreted both ways.