You can't paint one side without it bleeding to the other usually so your best bet is to agree to paint a side each and split the cost of the stain.
Then again you might find like in our case that you own the fence on e.g. the left of your garden and your neighbour owns the one on the right
I used Cuprinol Fence Life Plus and yeah, it's pretty watery. Probably like the consistency of a 50:50 mix of emulsion paint and water