Dual Sub-woofers paired into two sets of speakers - connection option.

@tunehead .... Do both. SVS and us....
It'll be interesting to compare/contrast the results.... ;)

Knowledge can be a terrible thing: 

Either makes' your day or ruins it. *L*

If your subs are powered, then the speaker level inputs will go to a line level converter before going to the sub’s plate amp. This will present a high impedance connection compared to your mains. 
 

Speaker A output to R/L mains and Speaker B output to R/L subs should be no problem.
 

Folks use dual mono subs driven off a sub or preamp out so they can position and adjust the subs to null out room nodes. For many systems this produces better results than true stereo subs. Lots of reading on this subject is out there.

A another consideration is high passing your mains to 1) better integrate with the subs and 3) take the bass load off the mains. I use this approach with great results.

good luck and enjoy

Here is an excerpt from Barry Obers white papers on sub integration on stereo bass (subs).

STEREO BASS?

Oh, and to touch upon "stereo bass" for a moment... there almost is no such thing. Going back to vinyl, every stereo vinyl record cut in the last 60 years has mono bass. It has to. If the bass were 180 degrees out of phase L and R then there would be vertical modulation and the stylus would jump out of the groove. Therefore most cutting lathe electronics have a "compatalyzer" circuit, that dumps frequencies below 160 hz into mono (typically a single-order filter, therefore 6dB/octave). You MAY have out of phase bass (i.e. "low frequencies") on a CD, but precious few producers/engineers are savvy enough (or care enough to even bother, since, typically, what's the point?) to make use of those sort of tricks. There are some EDM dubstep dance trance psychedelia eurotrash electronica club music releases where there are bass tracks where there is stereo bass in the form of something like 24 Hz in one channel and 24.2 Hz in the other channel; therefore you get an air pressure differential which travels around the room. Cool! In the above example, the "traveling pressure differential wave" would take 5 seconds to go back and forth around the room. If you're a really bored or obsessive techweenie you can have a lot of fun with this - we played with this phenomena at Moog Synthesizer as far back as 1969. Expect to either make your listeners nauseous or to watch their heads rotate on their bodies not unlike the effect in the movie The Exorcist.

Basically there is no advantage to stereo bass.

As far the connections you are describing, here is what Barry has to say about speaker level connections.

Simply connecting a sub to existing mains speaker (or amp) terminals is the WORST POSSIBLE WAY to do this.EVERYTHING scientific and acoustic about this method is wrong, from the additive delay issues to the back EMF of the mains affecting the LF signal.

If you still want to try this connection, I think you will be okay, but I agree with Tony1954, I would consult SVS.

Thanks to all.

@audiorusty Thank you for that. I’m now feeling more like this experiment is not going to yield any appreciable gain. To that, I did hook up one set of speakers (speaker A) from the speaker binding posts to each sub and did not hear any difference. It was an experiment I was interested in for awhile since I purchased the dual subs. Now I know for my self and my own ears.

I did not hook up the second set of speakers (speaker B) to the same dual sub’s as I am now convinced I will get no advantage, just possible loading problems.

Thanks to all again who replied, you knowledge and in insight is much appreciated.

PS: I did contact SVS but have not heard back.