You should be fine combining them. It shouldn't damage the 2151 to reverse polarity on them, and I believe they are designed to behave normally in such a situation, as in not resetting when polarity is reversed. I've accidentally fed negative 24 volts to a 2W-B and it behaved normally, I would expect the same from the 2151 since it's more or less the same product in a different shape.
But it doesn't just reverse polarity, I think it also increases the current on the IDC because the sounders need more current. I'm worried that it could fry a 2151.
Edit: it looks like it switches directly to aux power on the IDC, essentially connecting the smokes directly to non-current limited aux power in reverse. I don't think directly connecting it would be a good idea. I think I'll just have to buy more wire.
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GE Concord 4
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