There are a few ways, the easiest way is via a hunt group... Create a new one, add members of managers phone and the alternative phone, set the recall point to the ACD group and set your advance timer to 10-12 seconds and your recall timeout to double that.
If you want this to happen all the time (every call 24/7) make a new System Forward Path that routes to the hunt group and apply it to the managers phone for all conditions and make it the first in the list of forward paths. If you want to be able to turn it on and off, you could use Call Forward Always to the hunt group, or make a system forward path that activates only on DND which allows them to toggle it in a single key press.
Could also do it with a multi-step System Forward path...
I'm sure there are more... but this should get you where need to be.