All of the larger online shops will have tandem cables as will any shop that specializes in recumbent bikes/trikes.
They are not particularly expensive either
Brake Pyramid Cable Brake 1.5 X 2667 (105") Steel Standard Universal Tandem
Gear Sunlite Gear Cable 1.3X2794(110)Steel Std Univ Tandem
I used both of these brands in the past and they work fine.

When you cut the housing to fit, be really careful to not crimp the opening in the end. That could be a source of your problem. Most cable linings are polyethylene or Teflon. Oil or grease should not damage either one.

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