There are many leads that there is no 50% reduction to do. Would darth revan in the 2nd slot apply deathmark on the enemy leader?
Would Hera cause Phoenix allies to share uniques?
Would GAS prevent revives?
None of these can be simply cut in half.
Many percentage in kits cut in half would be a benefit. Mon Mothma: assisting allies have damage reduced by 90%. Cut that one in half and the rebel fighters are smashing way more damage out there.
This idea would require major work on nearly every lead ability, and would be a nightmare to try and balance.
The game already has a way of having 2 leads active on the pve tables, why not just use that? Have a chromicron or something that activates as if they are in ally spot.
Still would be a balance nightmare, so no thanks from me.