Double stroller or not?

Tk

Hi I’m 6 weeks pregnant with my second and by the time this baby comes out my son will be 2 yo. He is now 17 months and his PT said he probably won’t be able to walk until he is way past 19 months. Should I buy a double stroller or a stroller with toddler seat on the back? Or should I try to make it work with the single stroller that we have, I hate for our stroller to go to waste cause it was an expensive one? I have a bad back and cannot carry my son while pushing the stroller. I’m trying to reuse most of the things we have. Then again with the pandemic and everything I’m not even sure how many times we will actually use it, but good to have the option in case I need to go out. I mainly go out by myself with my son cause my husband is super busy and never leaves the house. Does anyone know how far a regular 2 year old can walk before their legs give in on them? I’m not even sure if my son can even walk by then, he can barely stand on his own now. Any advice would be appreciated. Thanks