Multi link is a way to configure a box with multiple modems to aggregate the bandwidth provided by multiple connections.
If you have an ISDN line, it’s typically configured as 2B+D, where B = 64k or so. Your router/terminal adapter usually (if configured) dynamically links and drops the second B channel depending on your traffic flow, so you don’t need multilink that way. (This is known as Bandwidth on Demand (BONDing)).
Typically, multilink is used with multiple analaog lines and modems. The provider at the receiving end has to support it, too.