Can i make a relationship between two relationships in the database?
Yes! You can make a new table which has foreign keys to both treatments and patient-records.

But be careful either in your model or in your code that you don't associate patient-records from a different patient with the patient associated with the treatment... using natural keys is one way to solve this:


treatments has a primary or unique key (doctor, patient) -- Doctor and Patient can be surrogate keys -- and both of these are foreign keys to doctor and patient, respectively.
patient-records has a primary or unique key (patient, medical-record) -- similarly, these are both foreign keys.
your new table has a primary or unique key (doctor, patient, medical-record) -- and there are two foreign keys: one on (doctor, patient) to treatments, and the other on (patient,medical-record) to medical-records.