It's simple math, folks. Didn't we all learn this stuff by freshman year? I bet you were one of the ones who went "I don't need to learn/remember this stuff, I'm never gonna use it in real life!" Well guess what...now it's "real life," and I bet you wish you'd learned/remembered it now, don't ya?
Since the one liquid is 50/50, you'll be adding the same quantity of PG as you add VG. You want to add "x" amount of each ("2x" amount of liquid, total) so that the resulting mix is 60/40.
(65+x) / (35+x) = 60/40 = 1.5
(65+x) / (35+x) = 1.5
65 + x = 1.5(35 + x)
65 + x = 52.5 + 1.5x
12.5 + x = 1.5x
12.5 = .5x
25 = x
You want to add X = 25%
each of PG and VG, or 50% total of your 50/50 liquid. Let's check, with an easy example of 100ml liquid so that 1ml = 1%:
65ml PG/35ml VG
Add 50ml 50/50, so 25ml PG/25ml VG
Total 90ml PG/60ml VG
90/60 = 60/40 = 1.5
Check!
Also:
90 + 60 = 150
90 / 150 = .6 (60% PG)
60 / 150 = .4 (40% VG)
Check, again!
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Edit: to clarify, you start with 100ml (or 100%) of the 65/35, then take 50% of that amount and add
that much (50ml in the example) of the 50/50 liquid. Or in other words, 2 parts 65/35 to one part 50/50, for any size "part."