If your mould splits in half, don't panic.

If it's a clean break you can pin it back together.

A quick sketch of the process,

So:

you want to minimise damage to the split faces. don't fit them together until you are ready to do something.

Drill your staple channels, and then flare the openings so they have a slight Y shape.

This way when the metal cools and shrinks slightly, they will pull the two pieces together tightly. 

You will need to close of the bottom opening of your staple. You can use another piece of stone held in place, a thick piece of metal, a bit of wood, whatever you feel comfortable with, so long as it doesn't make a perfectly airtight seal. (Remember, air still needs to exit)

You get one shot at getting this right, so make sure your metal is good and hot.

And here's one I've done. Here you can clearly see the fracture line, shown where the colour of the stone changes.

this mould split into two pieces..

And Showing three of the Staple ends,

And both ends in turn

this mould still works perfectly.