I don't ride a tandem, but:
I have found on a regular mountain bike, the rear tire wears faster than the front. It is somewhere near two back tires to one front tire.
On my fat bike, the back tire wears only a little faster than the front. It is somewhere near four back tires to three front ones.
There are probably other factors that also influence this. For example, where I ride the fat bike there are few hills. There is also a large proportion of dirt tracks. It seems tires last longer on dirt tracks than solid surfaces.
I use the front brake much more than the back in both situations.