The pack I'm developing is using the smeltery as a gate to bronze age technology while removing its ability to double ores. I'm wondering if you can allow the melter to alloy ores and hold more than one liquid so we can use that to gate the bronze age instead and allow the smeltery to double ores.

Nope. That goes against the while design of the block.

Design apart, it is one tank and it is not trivial to just add more tanks and alloying.

Consider finding a mod with a vat block or adding a custom item crafted from tin and copper that melts to bronze