Alaikum salam
Well i am not a scholar and couldnt find smth to asnwer your question properly so i will tell you what i do. You always have 2 options when doing istikhara. Either you do smth or not.
Lets say you want to buy a house near the river for example.
You make istikhara. "...If buying this house near the river is good for me, for my life and my akhira etc etc...and If buying this house near the river is not good for me,keep it away from me etc etc.. "
But you also thinking of buying a house in the mountain, or in the city, or in the village.
So you make the same thing with each of them, istikhara prayer separately.
Again this is what i do, and i found success with it. I dont know whats the best thing to do in islam. I couldnt find smth related to it.