How to connect monoblocks to a single REL sub with ONE RCA input

I have a REL S/812 and two PrimaLuna EVO 400's as mono blocks.

this is what I received from REL

Thank you for contacting us, and for choosing a REL Acoustics subwoofer for your home audio system. While some older REL designs did allow for a single subwoofer to simultaneously be connected High Level to two monoblock power amplifiers at the same time, with all current REL models including the S/812 a single subwoofer should not be connected High Level to two monoblocks at the same time. Doing so will in most cases result in a significant ground loop hum which can be near impossible to eliminate, and in a worst-case-scenario can result in damage to the subwoofer and/or the monoblock amplifiers. 
 
When using a single REL subwoofer in a system that features monoblock power amplifiers, the best option will be to connect it to open outputs on your preamp using the Low Level input. This will allow the subwoofer to safely draw signal from both the left and right output channels, and since the Low Level input feeds through the same filter circuit as the High Level input you will still have full control over the S/812's Crossover dial for tuning it's output to blend with your main loudspeakers. 
 
In this system it is also possible to connect using the High Level input to one of either the right channel or left channel monoblock amplifier. This will of course result in the subwoofer only receiving either left channel or right channel signal, though since sub-bass content in many popular recordings is centered in the stereo mix, meaning that it is common between the right and left channels, it will still be possible to achieve a seamless blend with your main loudspeakers when connecting in this way. When connecting in this way to the Primaluna EVO400, please refer to the following instructions:

• Twist together the red and yellow wires of the subwoofer's High Level cable, and connect to the 8 ohm positive output terminal on one of the two monoblock amplifiers.
• Connect the black wire of the High Level cable to the negative output terminal on the same monoblock amplifier. 

 
I hope that this helps, but feel free to let me know if you have any additional questions or concerns. 

NEVER use high level outputs to drive a subwoofer. It's a Band-Aid that should be banned.

According to the REL S/812 manual, it has two low level inputs. You can simply Y off the main pre out if there are not two sets of outputs

Ideally, you should roll the lows out the mains with a passive HP filter. Doing so will tighten up the low end and effectively double the apparent power of the PLs

That being said, you should flog the REL and get a pair of subs with real controls. IMO, REL markets bling, not good bass and are not serious about proper system integration due to their dearth of controls making them impossible to integrate properly. e.g. 180° phase inversion is equivalent to moving the sub about 25 feet @ 40Hz. Fixed XO slope will only integrate properly in a tiny subset of rooms and systems. 

Martin-Logan X series and JL Audio are excellent offering full control.

Sorry but more than one sub in one room seems overkill.Can also lead to standing waves and cancellation not only not doubling bass ability but cancelling each other out in some frequencies.. All seems a bit of a problem really.

@linndec Don't be sorry if that's the way it seems to you. You're mistaken. At least for my room. And you understand that generalizing about this is pointless, since the room's particular dimensions are critical to the number of subs. 

I've googled it for you, just to get you past reflexive opining.

https://mehlau.net/audio/multisub_geddes/

https://www.svsound.com/blogs/subwoofer-setup-and-tuning/75040195-why-go-dual

https://www.audioholics.com/subwoofer-setup/two-subs