First off how are you resting the rifle to group it off the bench? If you are letting the rifle rest on the sand bag, on either end, you are doing it wrong! A double rifle should never touch anything but your left hand, right hand, and your face, and shoulder when being fired. These rifles are designed to be fired off hand. It is OK to rest your hand on the sand bag, but hold the foreend the same way you would if shooting off hand. Do not rest the butt of the rifle on a sand bag either, Try to sit as streight up as if you were standing to shoot. A double rifle must absolutely be allowed to recoil in a natural manner, if it touches anything but the shooter, it will not regulate properly.

Now that you have the proper way to hold the rifle for shooting, the fact that the barrels are crossing, the left barrel shooting on the right, and low, and the right barrel shooting on the left , and low indicates the velocity is too high. It is not getting enough barrel time (bullet not staying in the barrel long enough durring the recoil arc)

If you take the barrel set off your Merkel, and place two empty cases in the chambers, with no primers. Lock the barrels in a padded vise by the lumps under the chambers, with the front of the barrels supported so that the sights are pointing at an aiming point @ 100 yds. Now look through the primer holes of the empty cases in the chambers. you will see the RIGHT barrel will be looking at a point that is LOW , and on the LEFT of the point of aim on the sights, and VICE-VERSA for the left barrel. When the rifle fires, the RIGHT barrel will recoil UP , and to the RIGHT, so if the right barrel is shooting on the left it isn't staying in the barrel long enough, so is too fast, the same but opposite is true of the laft barrel shooting low, and on the right. The speed needs decreasing.

What you need to do is shoot 3 rounds from the right barrel, then holding the same target picture fire 3 rounds with the left barrel. Now measure the distance from the center of the left barrel group to the center of the right barrel group. Half way between the centers of these two groups is where the rifle is actually pointing, regardless of where the sights show you to be pointing. If that center measurement is on the left of the aiming point on the target,the rear sight needs to move slightly to the left, till the center of each group is an equal distance from the aiming point. Don't move the sight very much, a little goes a long way.

Actually, what you need do is get the two groups shooting at the same place, by adjusting your load, regardless where it is hitting the target. Once you get the proper speed with the bullet you intend useing, the two groups will make a composit group, by fireing six shots three from the right barrel, and three from the left barrel. These six shots should be fired in the order of rt, lft,rt, lft, rt, lft. Now that you have the composit group of both barrels, you can adjust the sights to the exact center of the group! You will have to hand load for a double rifle, if you are to have any hope of makeing it shoot to regulation, and once you get the proper load, use nothing else, no matter what you hunt with the rifle. With your 30-06 a good load with you 180 that developes about 2450 fps, will likely be the one that will regulate. There is no need to find a low pressure load for this rifle/cartridge combination, as long as you stay within the 2450 to 2550 fps, the pressure will takee care of it's self!

I strongly suggest you get the Graeme Wright book, and read it cover to cover, then read it again for understanding. It will not likely have anything in it about the 30-06, but the manchanics are the same for regulateing any cartridge in any S/S double rifle. You may as well set up to hand load now, before you go any further, because factory is of no use to the man shooting a double rifle, chambered for a cartridge that was not designed to be shot in a double rifle. All those different bullet weights, and shapes will do you no good. A double rifle is designed to shoot a particular load to a given point, change that load and she will show you she doesn't like it, by shooting all over the map.