Hi L.,
If you're thinking of a double running stroller, keep in mind that for the first few months, your new baby will be to young to ride in the stroller while you run (not enough head support). After that, your son might be even more reluctant to use the stroller!
As for a regular double stroller, my son was 23 months when my daughter was born. I had him in the stroller and her in a sling. Now that she is 5 months old, she more and more often rides in the stroller and my son walks. The occasions when I would really have needed a double stroller (like to restrain them both during my doctor's appointment!) have been very rare.
As my son walks more and more and enjoys it, I am not sure if I will invest in a double stroller when my daughter outgrows the slings (still several months to go). At that time, the double stroller would be convenient especially when it's my son's nap time and we're on the go or even for time-outs. So, I'm not sure it will be worth it.
It depends a lot how often and how long you walk (I walk everywhere!)