Your bit about the 2 NIC-bridge is just a fancy way of saying, "No", isn't it?
Well ... if you had one computer that needed access to it and the Amplifier was sitting on the Guest network side it would work for that computer and any other computers with dual NICs connected that way, and anyone connecting to it on Guest network would have access to it. But being on the main network scope and tunneling to that amplifier on the guest network, unless some sort of port forwarding tricks are around to tunnel to it from outside the guest network, it wont be accessible.
If the Guest Network Router has DMZ support you could place the Amplifier with a Static IP Address on the DMZ, where the DMZ is your Main Network( This is not setting the Amplifier up on the DMZ of your Main Router or that opens up security vulnerabilities as well as gives access to amplifier to all who find it on the web.) However to access the Amplifier on the DMZ of the Guest Router you would likely have to change the gateway IP address of the Main Network or those devices that will access the amplifier to the IP Address that the Main Router has given to the Guest Network Router so if the Guest Network Router is given a static IP of 192.168.1.2 and the main network router is on 192.168.1.1 and the Amplifier is on the Guest network at 192.168.2.15 then from the 192.168.1.x network devices there would have access to it at 192.168.2.15 as the request to the gateway would be a request to the Guest Network router which will direct the traffic flow to the Amplifier on the DMZ.
I haven't tried what I stated above before but in theory I believe it would work, however those devices with the gateway pointing to resolve through the Guest Router ( I feel wont have internet access until their Gateway IP addresses are set back to what they were on the main routers network.)
So to make the connectivity work between Main and Guest networks for the Amplifier, its my assumption that you would lose your outbound to the internet for devices connected this way until their gateway IP's are changed back to the outbound to go to the WWW vs further up stream within the home networks.
If this all seems like a lot of work to try then... No would be the answer I guess. I've never set up a device this way to make that work, and in theory it should work through DMZ but no guarantee. But it is secure if only Guest Network DMZ setup as the DMZ is local to main network.