Sure, they certainly can be, but would require extra circuitry if not using a balanced signal.
The better and cheaper option would be to use the next level of Hypex amplifiers, the UcD700, which I am certainly happy to provide.
The reason I am hesitant to offer a bridgeable system like this is because each amp channel only sees half of the impedance of the speaker. So, if your speaker dips to 3 ohms or so, each amp only sees 1.5 ohms.
Bridged UcD400's would struggle a bit here, but a single UcD700 would drive a 3 ohm load all day long.
If you are running 8 ohms, no problem at all, but the UcD700 would provide similar power at a lower cost.
-Paul