In reply to Fraser kid:
> Unless, would I be able to use two slings that were looped over two separate spires/spikes, tied together at the equalising point with an overhand knot, with the carabiner threaded through the loop for the climbing rope?
That sounds like a quick option (as long as you're threading the carabiner through both slings so they're independent).
If you have a length of static rope, you could use that relatively easily as well. Tie a bowline with one end of the rope around one spike, bring a bight of rope to where the carabiner will sit, then wrap over the second spike, and back - tie an alpine butterfly with the loop around the second spike, then tie an isolating knot where you left the bight and clip it.
Or tie a bowline around one spike with one end, then a bowline around the second spike with the other end, and with the too-long loop between them - take a bight where the carabiner needs to go, and tie an alpine butterfly on the bight and clip it.