this is the split-ring-key, by scott amron. another 'why did no one think of that before' idea, i think it is excellent!
Carry your keys on your other key. Real working key blank. Key blank can be cut by any key cutter to fit KW1 or SC1 keyways.
and only $7 for two!
link to amron experimental (where there are lots more nice things to look at!)