I don't see the relevance of either version to your training, so just do whichever you enjoy most.
Rip,
I used to do seated dumbbell shoulder presses as an assistance exercise, but I was wary of doing that knee kick thing to get the dumbbells in position after getting my rotator cuff repaired, and tired of asking fellow gym goers to hand me a dumbbell (they'd often put their hand in the middle after I demonstrated that they should hold the dumbbell from the outside so I could grab it in the middle, etc.). So I started doing them with one arm recently, so I can use both of my arms to get the dumbbell in position. Then I hold on to the bottom of the seat with my non-lifting hand for stability.
What I've noticed watching myself in the mirror as I do these is that I get more range of motion out of my shoulder joint pressing one dumbbell at a time, because, without the interference of a second dumbbell, you can lock out the press with your arm straight up at a 90 degree angle to the floor. Does this make one armed seated dumbbell shoulder presses superior to the two armed version?
I don't see the relevance of either version to your training, so just do whichever you enjoy most.
How about picking up the dumbbells standing, cleaning them into the rack position, and then sitting down with them? Or just do standing presses?
From Practical Programming For Strength Training, 3rd Edition, by you and Andy Baker, p. 107:
So, dumbbell presses would appear to be relevant, and we're left with the question of the efficacy of the one-armed version versus the two-armed version. It might be worthy of further investigation. My guess is, all else equal, most will be able to lift more with the two armed version, but the ease/safety of getting to the start position, and the added range of motion of the one-armed version, may make it superior for many lifters, particularly older ones, or those with prior shoulder injuries.There are times when non-barbell-based assistance exercises are appropriate. Barbell exercises are very stressful, and there is only so much that a lifter can recover from within the training program, and only so much he can do within a given workout. [...] Dumbbell presses at all angles (flat, incline, and seated) can be useful tools to add mass to the shoulder girdle.
If you are using dumbbell presses as assistance exercises, then yes, they are useful. That would imply you still program the barbell press for progress as well. Absent of this, that quote is misapplied.
I would suspect that your more perpendicular position with one arm is due to either a slight tilt of the torso, or when you using 2 dumbbells they either hit in the middle or you stop short of them hitting in the middle. I'm betting on the former since you indicated you hold the bottom of the seat with your non-lifting hand. In my opinion, it would be difficult to do that without pulling to some degree, which would tilt you a bit. That said, if you want to use dumbbells, and find you can lift more or more safely with 1 at a time vs. both, have at it.
In general, though you will find you can lift more with a barbells, and it is easier to program progress since you can microload 2.5lb or lower spread across both arms.
To add mass, not to supplement the basic barbell lifts. The quote argues for additional dumbbell work to add volume instead of doing extra sets at a lighter weight, not for the substition of barbell lifts with dumbbells lifts.
There is no question to the efficacy of the one-armed version versus the two-armed version. The barbell lifts are more efficacious by virtue of being heavier and harder to recover from, thus un-supplementable. Like Mr. Gottstein says, the quote is misapplied.
I do some dumbbell presses as assistance work, mostly offseason when I'm not directly preparing for a meet. Obvious caveats that novices and early intermediates don't need them etc...but if you're at the point where they're actually useful for you, per the quote from PPST above:
1. You may be surprised how much you can "clean" with the dumbbells - don't discount this automatically without trying. This isn't and doesn't need to be a technical "clean." You'll use a combination of legs and arms, and yes there will be some arm bending in a curlish motion to get the bells up there. But it's not really that hard. When I was doing DB presses with 100 lbs, yes, it became somewhat unwieldy to clean, sit, and then press overhead. But for most people at most weights, it's not a huge issue.
2. I agree with Gottstein above that your additional ROM is due to either a torso tilt or hitting the bells together at the top of the lift. I don't think there's anything wrong, per se, with using a very small amount of torso tilt when doing a 1 arm DB press but nor is there anything specifically beneficial per se about it. I don't meet my bells in the middle at the top, nor do I press them towards each other. I press them more or less straight up and shrug into lockout, just like with a barbell.
I suppose I could give the clean a try again, and yes, I was touching the dumbbells at the top when I pressed both of them. For the context of how I'm using dumbbells, this is my current program:
Workout 1 (Monday or Tuesday):
Main barbell lifts: Deadlift and Bench
Assistance work: chin-up grip lat pull downs and seated dumbbell shoulder presses.
Workout 2 (Friday or Saturday):
Main barbell lifts: Squats and Press
Assistance work: weight-assisted chip-up machine and dumbbell bench presses.
You two must be morons if you think I've read the books but don't do barbell presses, particularly when I said in my initial post that I was using dumbbells as an assistance exercise.