I use a carrier so I can take her with me. If I needed to feed her I'd get the older one to come back to a table with me and get them a snack or something while I nurse the youngest
I would time it so that I fed immediately before leaving if it was going to be 40 minutes. Or get a wrap so you can hands free feed. But with your older one not running away constantly that will make it easier for sure. You will definitely manage! The more you do it the easier it gets.
That’s exactly what I did! I find an activity I know my oldest will stay engaged with for a while and sit on the floor and nurse or stand next to her and nurse while she plays. I also have found places like that are full of other moms who have been there and have jumped in to help when my toddler runs off to another area.