When I play, the only way I can only play only one of the pair of strings is to pick so softly that the the pick stops before it hits the second string. Either that or by placing the pick kind of under the string and picking upward so that the pick never touches the other string. In normal picking, (and I played guitar for 40+ years) I naturally play both strings and I don't feel like I'm doing anything different from when I played guitar, as far as attacking the strings with the pick is concerned. All that to say, I think if you keep your picking motion parallel to the plane of the strings and follow through a little more, you'll find yourself picking both strings.
"I thought I knew a lot about music. Then you start digging and the deeper you go, the more there is."~John Mellencamp
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"IT'S T-R-E-M-O-L-O, dangit!!"~Me