The Double Fisherman's Knot (Grapevine Bend) is an excellent and reliable way of joining two climbing ropes. It can be used for a full rope-length abseil; after which it is still possible to retrieve the rope.
Tie a "Fisherman's Knot" instead of unity candles....“true lovers knot” is the strongest there is; it will not break, and will only become stronger when under pressure. Although the fisherman's knot is one of the simplest knots to tie, it is also one of the strongest. The rope itself will break before the knot will."