lorax

Here's how it works for me - I have had bananas much closer to an Avocado (and uphill of it to boot) than yours is to the planter, and I was never able to drown it, regardless of what kind of water I was giving to the bananas. That garden was in tropical semidesert conditions, and in the peak heat I was soaking three times a day for an hour each time. So long as you have good drainage I can't see there being a problem; Avocadoes are actually thirstier than most people think.