Terry
I beleive Warner was the first to use a two peice bullet in a famous match with Perry, prior to that I beleive slug gun bullets were either shot as cast and/or cast and swaged   
one peice alloy PP bullets. If you mean two alloys cast in one mold I don't know, I tried itmyself without success.
Barry

scheutzenmeister,
I wish I could answer your question, but I can't.
to muddy the waters, different wadstacks can and do affect bumpup.
softer ones tend to reduce it.
an airgap when breech seating has less bumpup than the bullet on the powder.
William metford struggled with antimony to harden bullets, and was plagued by what he called "ripening" of the alloy, or it changing hardness with time. dan Theodore also did work on this.
at the moment I have enough to do with the simplicity of lead/tin only, as was the habit in America in the day.
I have gone as hard as 14:1 with good success so far, but must lay my cards on the table and say that experience has driven me to paper patching.
as a long range shooter, my bullets must of necessity have low drag noses, and every greaser I have tried will bump the nose into the rifling to some degree even at 14:1, up to where the patch would normally go.
we must remember that no normal bullet leaves the barrel looking like it did when cast.
in my experience lyman no 2 has been more of  a problem for accuracy than when cut with an equal quantity of lead using greasers. I have not recovered any bullets to study nose slump.
keep safe,
bruce.

Indeed, re. 
"ripening of the alloy(w/antimony)"
rule of thumb I use for WW etc.
20 days ripening time to develop full hardness, then it is stable for a 'long time"-based on NRA studies in '60s-see NRA Cast Bullet Handbook.
Has worked for me for the last 50 years.
beltfed/arnie

Lead/tin alloys change hardness,too.......
Tho, granted not as much
beltfed/arnie

The electronics industry has spent a lot of time and money studying alloys that are similar to what we use.  If it's your thing, you can anneal your antimony bullets to restore their hardness to the same as when they were cast .  So the stability issues can be solved with some effort.

From what I understand, the low antimony ( 1.5-2%) alloys that are attributed to Dan were actually previously used by Michael Rix.  I believe Dan became interested in them after Michael set an NRA record with them.  Dan did spend a fair bit of time researching them.  From the stuff Dan sent me, they were less good than 16:1 lead/tin for bore diameter bullets, but I these alloys obviously have potential for groove+ diameter bullets based on Michael's results.

Chris.

Well, I just got back from fishin' and read this and I give one. Here's why...
I inherited a lot (a whole lot) of bullets that were cast in 1954. Sixty years plus. Cast in wheelweight material of the time, they are ripe in antimony. I've been shooting them a little at a time for twenty years when I feel like tossing some gascheck stuff downrange. 
Never noticed anything bad from 'em. Shoot just swell. 
I'm over seventy, and I hope I'm gonna outlast those damn bullets.

Schuetzenmiester, you are right. Folks tend to overthink the alloy business of cast bullets. 
The best caster, and shooter of lead slugs I've ever known mined the berms and used the product. Whatever it tended to be. He was little concerned with what it was exactly. He just added "about this much" pure lead and some tin for his bullets.