Borrowed Life

Starting with 2 possible targets, when this spell is cast on two (or more, at higher levels) living, willing targets chosen by the caster (this can include the caster), then the targets of the spell pool their current hit points (including hit point increases caused by wild shape or similar effects) and maximum hit point values. All selected targets must be within the spell's range.
Any damage received by the persons affected by this spell reduces the pool of hit points instead of the hit points of the person(s) the damage actually hit. Healing one of the persons affected by the spell will replenish the hit point pool, up to a maximum of the sum of each person's hit points. If multiple participants are hit by the same damage (such as an Area Of Effect spell or breath weapon), the hit point pool is reduced by the total of the damage received by each of them as individuals. If the hit point pool is reduced to zero or less, standard incapacitation or death rules apply to all affected persons, and each must make the required stabilization checks, or receive healing.
This spell only affects hit points; resistances, vulnerabilities, immunities, shape-changed abilities (that do not affect hit points), special abilities, and the like are not shared. Potions, curses, poisons, or other effects that do not affect health will only affect the person that received the effect, although any damage done by that effect will be taken from the hit point pool. If a person affected by this spell has their Con reduced by an amount sufficient to reduce their hit point bonus, the total amount their max hit point value loses will also be lost from the hit point pool's maximum. If a person affected by this spell has their maximum hit point value increased, the hit point pool's maximum hit point value is also increased. If a target of the spell falls unconscious for some reason other than hit point loss (such as a sleep spell), the other participant(s) will remain conscious. If the caster falls unconscious for any reason, it will break the caster's concentration, and the spell will fail as if it had reached the end of its duration.
If any or all of the targets of this spell are unwilling targets, then each unwilling target must roll vs. the caster's spell casting ability DC to resist. There is no effect on any target that succeeds their save. If all targets succeed, then it has no effect and the spell slot is consumed. If at least 2 targets fail, then the spell is effective and operates normally on the failing targets only. Any resistance to magic abilities held by either target will apply to that target only.
At the end of the spell's duration, or if the caster chooses to end the spell's effects, the remaining hit points in the hit point pool will be divided evenly between the persons affected. If a participant would gain a number of hit points higher than their current maximum hit point total, the excess hit points will go to the other participant. If, possibly due to CON bonus loss or other effects, there are excess hit points after both participants receive their current maximum hit point total, the excess hit points are lost.