Hi @Sohini Kar 

Parallel approval cannot be achieved using a single flow. To implement this, you need to create parent and child catalog items along with corresponding flows.

Below is the community response I provided for a similar requirement. PFB

https://www.servicenow.com/community/developer-forum/servicenow-flow-designer-approvals/m-p/3207671/...

Let me know if you have any queries.
Regards,
Siva