I'm making this egg free brownie recipe.

https://www.munatycooking.com/chewy-egg-free-fudge-brownies/

Unfortunately, I have only one pan and I need two round layers of brownie. So if I'm going to bake them in the same pan in two batches, how long can I leave the mix for the second batch out? 

Thanks in advance!

it'll be ok on the counter while you wait for the first one to bake off -- no worries

but you could mix each batch separately too -- just hold the second batch of ingredients in containers -- hold the dry and wet separate