How do I go about acknowledging my sources of funding when I am supported by 2 different grants equally? I am a postdoctoral supported by 2 grants (say A and B) with 50/50. Those 2 grants are owned by different PIs.

I would happy to have suggestions for the followings:


When I finish a paper for grant A, am I expected to acknowledge both grant A and B, or A alone?
Am I expected to put the PI of grant B as a co-author? Contribution could be ignored here. I can always just send the paper to the PI and he/she could contribute to critically review the manuscript. So I am wondering am I expected to do that. 


What is the obligation and common practices for these? Thanks.
You should only acknowledge the grant(s) on which the publication was actually based, i.e. where the funding/time/equipment came from. 

(Co)Authors should only be people who have actually contributed to the publication. There are already some good answers to this part of your question, see for example this one related to PIs.