Let's see...

- Parasitic aliens who forged a bond with humans (Parasites and Mecha-Ude)
- Our protagonist (Shinichi and Hikaru) gets fused with a unique kind of their species (Migi and Alma)
- They fight some baddies (enemy Parasites and the Kagami Group & some rouge Mecha-Ude hosts)

Contrast is that Parasites are organic while Mecha-Ude are mechanical creatures. Also, the female lead has no powers in Parasyte while she does in Mecha-Ude.