I believe in KISS. My own solution to your dilemma (yeah, I thought of it too) is to keep the magazine loaded with buckshot with one space in the magazine left empty and the SideSaddle loaded with slugs only. The gun is kept bolt closed on an empty chamber, action cocked and safety on.
Any necessary loading is always done through the loading port, the same way every time, as an established habit- ONE habit, one routine for loading, one place to put live ammo all the time (not two, loading port and ejection port).
Need a slug? Load one in the magazine on top of the buckshot, remember LIFO* (last in first out), and run it into the chamber. Need another? Same deal as before, only this time you will eject a live round of buckshot unless you took time to load another slug before you fired the first one and immediately ran the bolt, as you should always do with a pumpgun by reflex.
Think you can count shots under pressure? Maybe you can, I sure can't. It's all I can do to keep stuffing in slugs when slugs are called for. I understand why Louis Awerbuck says he isn't smart enough to run two kinds of ammo in a shotgun so he just uses slugs all the time. I'm certainly not smarter than Louis, just more stubborn.
Whatever you decide on doing, you need to establish it as a routine and then PRACTICE IT a lot. And IMHO the simpler the routines the better.
On the select slug drills on the range? Set up two targets, one for slugs and one for buck, and have a range partner behind you call the targets randomly for you to engage with the round designated for that target. Use numbers or shapes or colors for different targets and designate one target for slug and one for buck, and start with the gun in your normal standby condition. Run the drill till the gun is empty and you get to the load one shoot one stage. And concentrate on following through on your shots and getting hits, too.
After you flail around on the line for a while stuffing shells into a hot pumpgun with a safety-conscious range partner yelling SLUG! SHOOT! SHOOT QUICKLY! SHOOT! BUCKSHOT! SHOOT! SHOOT NOW! SHOOT! from behind you on your support side, you'll begin to sort out running the gun under pressure, however you set up your routine. If you've never tried it, you will be surprised how much pressure a simple drill like this can bring to bear. But make sure both of you have your safety habits well engrained in doing drills like this, and make sure you keep your muzzle downrange at all times. Even when you get frustrated...
Of course, IMHO if you can arrange it so the person doing the yelling is a certain diminuitive South African who runs Yavapai Firearms Academy, so much the better. Or Randy Cain, or any other really good shotgun instructor.
Louis says a shotgun is a thinking person's weapon. He isn't kidding.
lpl/nc
*Opposite of LIFO is FILO- first in last out.
