SWMBO says that lining are normally stitched to the top of the curtain so need to be the same width. She suggests that you could hang the lining from the same rail with hooks in between those taking the main curtain.
So twice as many runners and hooks and alternate ones for the lining and the others for the curtain. My practical comment would be to ensure that the combined width taken up by the runners will allow the curtain to open wide enough.
Separate linings have a hook tape, into which you insert asterisk- hooks to connect them to a hoop on the bottom of the curtain's slider hooks.
That is the fun bit, a 90" lining restricts a 109" curtain to 90" open. The 90" limit is bigger than the 60" closed curtain span, but I can not see how the lining will "pleat into the curtains properly" when the curtains are open.
Nothing else for it but tracking down custom curtain linings with proper hook tape. Surprised the ready made stop at 90" wide which is not really wide enough for many bay windows.
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