I bet you could but in that mode it's basically an l2 bridge from WAN to LAN. If you attach the WAN and LAN ports of two bridge mode MXes to the same respective segments you will have created a loop. You then have STP block one of the ports so only one link is effective. Notably warm spare mode doesn't add any value as you get the same effect by just hooking up two different MXen. The stats are aggregated together, I suppose.
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