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
