Ever consider the herbie as was suggested? You wouldn't need to drill any more holes if you could hard plumb your return line behind your tank and over the top so you can use your existing return hole for a herbie setup (adding to your durso). Depending on your setup, may be worth the peace of mind. Can google "diy herbie overflow" it or ask around here and get feedback.
Plenty of horror stories on the net about someone with one overflow having theirs clog (causing their sump water volume to end up on the floor). Life getting stuck in there, or something else going wrong, etc.
I converted my one overflow tank to this peace of mind setup (except instead of a herbie, i did a bean animal fully using my 3 holes as drains). As I kicked a half-full 5 gallon bucket over once, couldn't believe how much cleanup I had to do...I shuddered to think of my 55G sump dumping to the floor...