Left and right subwoofers with integrated

@millercarbon Thanks for the quick and clear response. My life is suddenly easier, which is a very rare occurrence these days.

     Two subs typically perform and sound about twice as good as a single sub.  You'll notice more solid and powerful bass with more realistic bass dynamics, a greater sense of ease, better detail and more seamless integration of the bass with your main speakers.  Two subs have a basic  advantage over just one primarily due to bass being cumulative in a given room; since there are two they can share the total bass demands between them with both operating optimally at outputs well within their limits.
     Here's the most useful advice I can give you for your scenario:

1. Most humans cannot localize (tell exactly where the sound is coming from) on bass sound tones below about 80 Hz, which means all bass below this threshold will most likely be perceived by yourself as mono.
     Since your integrated only has one set of l+r preouts and each of your subs requires a l+r channel input, however, I believe the connection method you described (using  a couple of y-adapters) would work well.  There's no real advantage or disadvantage of using this dual y-adapters method.  You're going to perceive the bass below about 80 Hz as mono no matter how you connect your subs. 
     The good news is that you'll still perceive a sense of stereo deep bass in your sound stage presentation due to the deep bass harmonics or overtones of the fundamental bass tones, which extend beyond 80 Hz and can be localized, being reproduced through your main stereo speakers. 

2.  If your goal is optimum bass performance at a designated listening seat in your room, I would also strongly advise against placing your subs in predetermined positions within the room, such as diagonally or any other convenient or predetermined locations. 
      The reason is that bass sound waves have an omnidirectional radiation pattern and their lengths are so long ( a 20 Hz deep bass sound wave is about 56' long), it's unavoidable that bass standing waves will exist at specific spots in your room no matter where you position each of your subs.  These spots are easily identified since the bass at your listening seat will sound overemphasized (a peak), underemphasized (a dip) or even non-existent (a cancellation) if your seat happens to be positioned by one of these bass standing waves. 
     The only methods I'm aware of, to ensure bass standing waves don't exist at your listening seat, are to position each sub sequentially using the 'sub crawl' method (google it) or using mics and other room analysis and  correction hardware/software.  I think it's important to note that the above are not merely my opinions but are based on acoustic research and knowledge that can be readily verified online.

Hoped this helped you,
    Tim

good advice given

stereo subs are wonderful, transformative of many systems

good for you!

+5
Sum the left and right at the sub. Crawl test your room for optimal or near optimal placement.

Enjoy

jjss49:" good advice given

stereo subs are wonderful, transformative of many systems

good for you!"

tony1954,

     Just for the record and your edification, there's no such thing as 'stereo bass' or 'stereo subs' beyond my description in my last post of the bass fundamental bass tones below about 80 Hz being reproduced by a pair of mono subs, which we cannot localize, and the bass harmonics or overtones of the fundamental bass tones, which often extend beyond about 80 Hz that we can localize,being reproduced by the main stereo speakers.  Our brains are able to associate the bass harmonics or overtones, that are above 80 Hz and coming from the main stereo speakers, with the fundamental bass tones, that are below 80 Hz and coming from the subs, and create the perception of stereo bass.
     I suspect some individuals confuse these stereo bass perceptions created by our brain's amazing associative abilities with the results of simply physically configuring their system with separate l+r channel subs in their systems.  They just don't completely understand the true cerebral associative forces at work and believe their "stereo sub" configuration is responsible for them perceiving stereo deep bass. 
     In my opinion, this is a common and understandable mistake which, ultimately, is not that big a deal.  No matter whether we configure our subs in stereo or mono operation, the final result is the same: we're all actually listening to mono deep bass from the subs and stereo harmonics from our main stereo speakers while our brains are doing the heavy lifting and creating the perception of stereo deep bass. 

Tim