Amalgamate

Two willing creatures you can see within 30 ft. that are within one size category of each other suddenly launch at each other at high speed. Upon converging, the two bodies merge into one amalgam. If either of the creatures understands you and also knows the spell, it can use its reaction to aid you, negating the spell component cost.

This merger physically alters your body so that you appear as a mixed version of yourself and the creature. You decide what equipment is absorbed into the fused being and what equipment remains available for use. These fused items are restored once the spell ends. The fused creature retains the size category of the largest creature in the amalgam. Once fused, the amalgam cannot be targeted by this effect for the duration of the spell. 

For the duration, the targets are effectively one in the same, and together share one turn. They remain in control of themselves—of the fused body—but both is fully aware of everything they are physically doing, and can see through each other's senses. While fused, the targets can communicate telepathically. They are both aware of any surface thoughts, knowledge, or memories of each other, but can purposefully conceal anything not at the forefront of their consciousness. 

For the duration, the fused target:

• Is considered a monstrosity.
• Have the combined maximum hit points and remaining hit points of the fused targets.
• Can make any ability check, attack roll, or saving throw using the higher of the targets' modifiers.
• If both fused targets share a proficiency in a skill, they always roll that skill at advantage.
• Can use all features and spells of the target, except for those it purposefully conceals from you. The amalgam uses the spell slots equal to the highest of whichever target. 
• If both targets are casters, the amalgam can concentrate on one additional spell. 

The spell ends if the amalgam is incapacitated or dies. Furthermore, at any time, either target can use their reaction to end this spell early. When the spell ends:

• The targets reappears in an unoccupied space of their choice within 5 feet of each other, and their bodies return to normal. If there is insufficient space, the targets are magically forced through whatever physical matter is blocking their reappearance in the shortest possible path to an open space, before reappearing in that space and taking 1d6 force damage for every 5 feet of matter through which they moved.
• All equipment on their person or absorbed in the transformation is returned to its original positions on its original owners.
• Damage dealt to the amalgam during the duration of the spell is split between the targets. If any damage exceeds the remaining hit points of one target, the excess damage is taken by the other one. 
• Any conditions, diseases, or similar effects that were afflicting the targets before the fusion are returned to it as appropriate. If any new conditions were gained during the fusion, roll 1d6. On an odd number, only the first target is afflicted. On an even, only the second target is afflicted.