Mixing the ingredients in the ratio 1:1 doubles the volume containing the fat, so the same amount of fat is present in twice the volume. That makes the concentration half of what it was before, that is, 3.5g per serving if there was 7g per serving before mixing.
There are roughly 28g in one ounce, so in one 12oz can there will be 336g. One serving contains 1/4 can (1/4c=1/4 can?), and 1/4 can contains 84g, so 1/12 of this measure is fat (7/84). 1/8 can of each ingredient is needed in the 1:1 mixture, so that's 1/24 of a 1/4 can of mixture. 1/24 of 84=84/24=7/2=3.5g.