The only way that you can use a main sail halyard as a messenger line is if you halyards are all mounted externally. But messanger halyards are usually only used to replace an existing line by sewing or taping the end of the line to be replaced with the new line that is replacing the old one by continuously feeding up the new halyard until the old one is passed through the mast sheave & down again to the deck.
It is very difficult to get 2 lines taped onto the end of one halyard to pass through the sheave at the top of the mast without getting jammed on the rather thin sheave pulley wheel. You can try it, but I wouldn't force it through if you experience a lot of resistance when hauling up the line to the top of the mast. Otherwise your liable to get the knot stuck at the mast head, & then you won't have a usable main halyard either.
Here's another suggestion entirely: buy a boom kicker rod which is placed from the boom & attached to the mast at a 45 degree angle, which supports the boom & does away with the topping lift all together. They are really not much more expensive, when you consider the cost of buying expensive 3 braided Dacron line running all the way from the boom to the mast head. Food for thought.