I use the CTOMS C-sling. It allows a quick and smooth transition with the option to tightly cinch the gun down so it doesn't shift. The same transition could be used with any 2-point sling I would think.
To stow: Starting from the fire position, with the sling around your neck like a necklace, reach under the sling with the weak hand and grasp the butt, let the barrel drop down and your strong hand holds the sling off your shoulder to allow it to slid more easily. Your weak hand guides the butt down to your weak hand side, your strong hand then grabs the cinch tab and pulls the sling tight before transitioning down to your side arm or to your grenades.
From stowed to firing: Grasp the butt with your weak hand, pull the release handle on the sling with your strong hand, pull the butt up and against your strong shoulder as your strong hand finds the pistol grip and raises it into a shooting position.

I run a single point bungee sling and on my chest rig i use a cpl d ring things one on the right shoulder and one on the left then i run the sling through the d rings so the bungee portion is between the rings and the lil rings on the sling that seperate the strap from the bungee are on the outside.
this way the sling stays in one position the rifle and its connection to the bungee can slide from one side to the other for shooting from the off hand and because the d rings keep the bungee off the neck you can put the rifle, barrel down over the right shoulder its not the most comfortable in that position but if u need to take a knee with a longer rifle in the front it will hit the ground(im short) i use a drop leg or belt mount on the right side for pistol and in most cases just hold the rifle in the off hand or drop it to hang on the front but sometimes, like in a building ill throw it over the shoulder for easyer movement in tight quarters. It works for me but i think id like one of those 1/2 point slings so the over the shoulder would be more comfy.

Mostly out of lack of sling, I don't shoulder my P90 when I switch to my sidearm. I fold my arm inwards and hold my P90 against myself with my foregrip hand and draw/aim my sidearm single-handedly.

I am unsure what this technique is called, but I see it used a lot. It's similar to what Snake does in MGS4 (though he holds the M4 with his firing hand and uses his sidearm with his weak hand.)

That's called the Snake technique.