I have two Orange 4200mAh 5S lipo batteries
1) 2* Orange 4200mAh 5S 35C/70C Lithium polymer battery Pack (LiPo)
(Initially it was 6s but i have removed one cell from each battery as it was dead)

2) Charger: ToolkitRC M8S 400W 18A
https://www.banggood.in/ToolkitRC-M8S-400W-18A-Color-Screen-Balance...

3) Parallel charging Board
https://hobbyking.com/en_us/hobbyking-parallel-charging-board-for-6...

For battery recommended Max. Charge Rate is 5C, I have never charged battery more than 2A.

My question is
1) Should i charge battery with minimum 4.2 A (1C) or is it ok to charge 8.4A(2C)?
2) When i connect battery to parallel charging board then, should i charge with minimum 8.4(1C considering two batteries) or use 16.8A(2C considering two batteries)?

Which is good for battery life?