In pre-amp setups like that (bipolar), it should also be known that since the circuitry is now having a double load, this effectivly lowers the MTBF for the components carrying the additional load, and increases operating temps, most medium/high end audio output preamps are mosfet driven, which would not suffer the same problems and are more reliable.
There are amplifiers that are used for amplifying long RCA runs which might be of use to you, but the ones I saw were audio/video and were 100 bucks. I know theey have them for car audio, so you could probably use one of those inline.
If the content of the outputs are identical, you would want to put only one of the outputs into the new inline preamp, then from inline preamp, plug into the devices to be driven.
This will reduce the load on the receivers preamp down to the load of one device, and the new amp will 'split' the signal to your devices.
Now, _if_ the 2 outputs content are NOT identical to each other, then you will need 2 preamps for each RCA run, but then you will still be overloading the preamp in this setup, but will have louder volume.