I have a need to rereeve my main and spinnaker halyards and would appreciate any advice on the easiest method. As both have to pass through two blocks built into the mast and the space is very limited I cannot simply tie the two ends together. I thought about stitching the two together end on but thought I would ask the experts before I get my halyards lost inside the mast.
If the pullies are in good condition you can use insulation tape or similar to act as a snotter to connect the two lines end to end. Wrapping the tape at 45 degrees and getting sufficient length of the lines for strength when hauling yet keeping it small enough to get through the blocks is the key to success.
Be sure your blocks are in excellent condition on the high load lines, even slight damage or wear is a good reason to replace the blocks to protect your new lines.
I forgot one other point. Leave a bit more length on the halyard if possible for future maintenance. Then in future it will be possible to cut some length off the shackle end of the halyard when the areas over sheaves start to fray. For cruisers, taking a few hundred millimeters off the line every year or so will increase the effective life of the line.
Don't cut the tales to the minimum length unless there is a good reason.