One data point: IIRC, a benchmark I ran comparing memory consumption for Lua coroutines (co-operative threads) used roughly 1.3-1.4x as much memory on 64-bit as 32-bit.

I'm not sure if 1.25-1.5x would be accurate as a rule of thumb, but 2x is almost certainly too high. (It will depend on the data structures etc. involved.)