A couple things you might watch out for;
- Careful of the BM1397 copper tops, they are connected to GND, which won't be at the same potential if you have the chips in series. ie, a common heatsink between two BM1397's in series is going to short them.
I thought the Bottom side is connect to GND and +Ve while the top has copper insulating substrate below it not connected to GND . I believe what you mentioned in your comment , if correct then , a common Heatsink for 2 chips is not possible. That's not a good design, i thought the top had great thermal conductivity but poor/no electrical conductivity at all due to insulating substrate below the copper top
Can you elaborate ?