Instead of doubling health (as seen in Banana Slamma), Rivendare Portrait simply adds its original health. This is important when you have two Rivendare Portrait trinkets.
Expected: Rivendare Portrait goes 1/7 → 1/28 (double, double)
Actual: Rivendare Portrait goes 1/7 → 1/21 (+7, +7)
Note this addition includes buffs. For example, if I add a +1/+1 banana, Actual: Rivendare Portrait goes 2/8 → 2/24 (+8, +8)