Neale, you could... but, measuring the precise amount of softer lead for the tip is a nightmare, and then having it harden too much before you pour the base. It has all been done before.
You can still buy moulds that cast the tip part first, by itself, then after they harden, you can place it in the full mould cavity, then pour the base section, or you can cast both parts seperately and epoxy them together, the weakest part will always be the join however, they come apart there when they hit something.
Also hard and soft alloys vary in dimension, even when cast from the same mould, and getting the two parts to fit correctly is nearly impossible.
Best to make a slightly softer bullet, load it so it doesn't lead, and hope for the best results possible.
Hollow points don't open up as readily as jacketed hollow points, at the speeds we would be driving cast bullets.

Even read a story in an old magazine last night about wire wound cast bullets, where the caster wound up copper coils from wire, placed them in a specially made mould, poured the core and had himself a semi-jacketed bullet, no leading and softer core lead. The downside is that he could only make 17 per hour!

Regards,
David.