How can we create memberships with two or more accumulated partial payments?
As of now, Partial payment is only possible through backend in CiviCRM. There is an open issue which extends it to be usable from live contribution page too. See https://github.com/civicrm/civicrm-core/pull/12319. To test/use this functionality, you need to perform the following steps -


Apply this patch on your site.
Create a pending/partially paid membership for a contact.
Select Default invoice payment page from civicrm/admin/setting/preferences/contribute?reset=1.
Navigate to user dashboard and you should be able to see a pay now link which points the user to complete this payment.
The pending amount is prefilled on the contribution page and user is able to complete the payment using this live contribution page. 


ToDo - On the last step, there needs to be a changeable amount field which user can use and pay the membership fee in different installments.