In reply to Taurig: The two solutions I would use for top-roping with a far away anchor in this situation would be:
With a static rigging line:
You produce a centre point with the static rope forming a 'V' from the two anchors with the centre point, ideally around 1 metre BACK from the belay position and cliff edge:
- With one end of rope tie a bowline around the large tree (with stopper knot).
- Measure out length of rope required down to centre point position (adding around 20cm for knot) and then back to 2nd tree.
- Clove hitch onto a screwgate on a sling around the 2nd tree.
- Tie a Fig-of-8 on bight (with small loop) to create the centre point (going back to adjust the clove hitch if required).
Now you tie into the end of the climbing rope, clove hitch your rope to the centre point and FOR THE FIRST TIME approach the edge of the cliff and then do any fine adjustment you require via the clove hitch, which should nicely be in reach. You are then ready to drop the end of the rope and belay.
Using just the climbing rope:
- Tie one end of the rope to the large tree with a bowline (and stopper knot).
- Clip the rope into the krab on the sling round the second tree.
- Put a prusik onto the rope going to the large tree and clip to your harness with a short sling to provide for your safety.
- Take hold of both strands of the rope leaving the krab at the second tree.
- Keep hold of these two strands and walk to where you are going to belay, allowing the ropes to slide through your hands (and moving the prusik if needed)
- Allowing for an extra 20cm for the knot, tie an overhand knot on the bight using both strands of rope to provide a centre point in the perfect position to belay from.
[If you have done this properly you will have an overhand knot with 4 strand coming out the bottom, on going to the large tree, two going to the second tree and one which is the remainder of the rope.]
- Clip yourself into the centrepoint with a screwgate to your belay loop
- Clip your belay krab and belay device into the centrepoint and you are then ready to drop the end of the rope and belay.
The first solution is one I would strongly recommend you study. As already mentioned by several other posters, using a bowline to connect one end of a rope to a far away anchor is a fairly basic and useful technique (not to mention being quick to tie and easy to undo after loading).
The second solution is admittedly using a rather more advanced technique on the sort more used by instructors, but it is UKC and you did ask...