Hi Krishna,
please read my blog posts:
Custom Workflow - Developer view - Part 1 - Idea and preparation
Custom Workflow - Developer view - Part 2 - Process Schema and Implementation
For the parallel approval you may need a special BADI implementation for agent determination.
BADI should return AREA to ITEM mapping table with multiple areas for items.
For example you want to have 2 parallel approvers for shopping cart with 2 items. Resulting area mapping table should look like:
AREA1 - ITEM1
AREA1 - ITEM2
AREA2 - ITEM1
AREA2 - ITEM2
AREA1 - for approver 1
AREA2 - for approver 2
Regards
Konstantin