I tend to think the opposite (just my opinion). I think they put pins on both sides just to make board design easier. Use whichever pins work best with your design. Gives you the option to find the easiest way in when routing. I'm probably wrong. I know lots of people disagree with me.
Of course I hook em both up anyway, just in case.
I'd really like to see some official notes on the matter though
I have had a number of inquiries and am interested myself to know the full extent of the requirements for connecting both sets Vdd and Vss on any 40 Pin PIC Micro. Are there situations where they are not required? Is there extended information regarding the matter?
Practical experimenting suggests that only one set of Vdd and Vss are required.
If device is having multiple power pins, then it will be advisable to tie the together. This
Is on of the good practice which needs to be followed in hardware application circuit design.
Refer the presentation of "power considerations" topic in the following web link: https://www.electro-tech-online.com/custompdfs/2007/12/power.pdf