Sharing a subwoofer with 2CH and HTs rigs!

Has anyone used an RCA switch box or passive pre to handle two RCA pre-outs from different amps to feed the LFE on a single powered Sub? Looking at the schiit SYS as an option. Any advice would be greatly appreciated!
I don’t think you want to use the LFE input for 2-channel — probably better to use either the L/R line level or speaker level inputs if available I’d think. 
Some (All ?) REL Subs have stereo inputs and a separate LFE .1 input and each input has its own trim.  
Thanks. I have an older Velodyne that has line level/LFE and speaker inputs with one dial for crossover level and one for sub volume, but I’m not worried about which input on the sub (I need to live with the RCA inputs if I’m gonna use two amps as separate inputs). Just trying to figure out if the switch box is an effective tool to split two amp signals into the subwoofer. Worried about sound degradation or it being a problem somehow for the sub or amps...
Is this the way to do it? Or are there other options?
I use my JL E112s for both home theater and music. The inputs are summed. So you can send the preamp to one input and the reciver lower frequency output to the others. This still lets you high pass the mains and play the receiver’s out at the same time. I have been doing this for years without issue. 
In the velodyne manual it expressly IN ALL CAPITAL LETTERS AND BOLD says do NOT use both low and high level inputs concurrently... 😕
Using the SYS, my thinking is that you could run a pair of RCAs to  input 1 from one amp and to input 2 from the other. Then run a pair of RCAs to the sub from the SYS output (or a y splitter to a single RCA if the sub has single input). Being very careful about having only a single amp on at a time, having both turned off when switching which to use.

I believe the same could be said for a switch box. 
Thanks, Mesch. That’s on point, and getting me closer to the answer I need - but your solution seems to say that the SYS or even any cheaper RCA switch box won’t keep the sub from being fried if the two amps happen to be on... Do passive switch boxes send current from both inputs even if only one is selected? I guess that’s the crux of my issue. It would really be the same as running both amps directly into the sub at once through different inputs and hoping I never make the mistake of having both on at once.... Does anyone know if there is a switchbox or rca cheap pre-device similar that will preclude having to worry about both amps being on at any point?? Or would the SYS and switch box be okay? Thanks a ton, guys. Flynn
I hope others will chime in on possible problem with use of a switch box. I would think any quality one completely isolates inputs. I doubt one would want one's sub to receive signal from two preamp outputs at the same time.

The issue differs between use while both amps on and use for selecting which audio vs. video amplifier to have on and selecting routing to sub without movement of cables.  Do you really need to have both amplifiers on at the same time. Is warmup a concern?

I am also interested in what others contribute.  
As far as 8 know the RELs are the only subs that can mix RCA stereo inputs and .1 LFE

“In the velodyne manual it expressly IN ALL CAPITAL LETTERS AND BOLD says do NOT use both low and high level inputs concurrently... 😕”

interesting, the JL I use just has right and left RCA. I have never used the high level speaker hookups. Maybe call them and see what they say.
If you have two I send the preamp to right on one sub and left on the other sub. Then send the LEF signal from the receiver to the open input.
This allows the mains to use the active crossover and you can still adjust the sub level through the receiver/processor.

I can’t recommend JL audio though. They are always breaking. The electronics in them are total junk (they sound good when they work....)
Hey, James. Velodyne is out of business. Talking to some pro audio guys about it to see what they say. They do this kind of stuff all the time, I would guess. 
Mesch- I certainly don’t need to have both amplifiers on, and don’t plan on it. My worry is that, by accident, at some point I WILL, and want to avoid that possibility! 
I’m also pretty sure most of the big box places like Best Buy do set-ups like this in their auditon rooms. I just want to be 100% sure as I really like this Velodyne sub and they’re not easy to find anymore.
I had use a Niles SAS-1 as a switcher to select output between a AVR sub-out and a active low pass xover from a two channel preamp to a subwoofer via RCA cable without any problem. Actually, any line level source switcher/selector can get the job done, and I believe if both source are turned on will not causing any problem as long as only one source is selected and connect to the sub input.
The only possible problem I can think of is most of the RCA switcher/selector only switching "+" side and the ground connection remain connected. In some cases, it will introduce ground loop hum. The solution is modify a xlr switcher to rca so both "+" and ground connection can be switched on or off.
Another possible problem is "cross-talk", it can be overcome by simply turn off the unwanted source.
Thanks everyone. A sound engineer I know thinks it won’t be a problem. I’ll be trying it later today and will post the results in case anyone else is interested. I searched everywhere I could think of on the internet and couldn’t find anything!