Every mold maker has a bit different hole spacing, and I believe LBT has the widest, which I set up almost 30 years ago and don't remember exactly, but it's about 9/16 inch. I did it because that thin web of metal between bullet cavities, which really gets thin on the larger caliber molds, is critical in carrying away heat from bullets each side of it. Being too thin is the cause of sunken dents in the sides of bullets, which happens most in cast iron molds, because of the low heat conductivity of iron. Steel blocks are some better, while brass and aluminum will carry heat away fastest.
In case you are wondering what that has to do with pouring two bullets at once, absolutely nothing. I just thought it might be good information for casters to have filed away.