With two layers of shingles, you already have a lot of weight on that roof. Given that rafters on old houses are often marginal, it is insane to add any more roofing on top of what's there. Architectural shingles weigh about 250 lbs/square, so if you do the math you'll find you're going to add the equivalent of a midsize car to the load on your roof. I don't think steel is as heavy, but it's certainly not weightless.
Another consideration with architectural shingles is that most are "metric" and won't match up with standard three-tab shingles, which will hurt their life. Best to go over three-tab shingles with new three-tab, if you're inclined to shingle over existing roofing. Personally I would NEVER add a second layer of roofing. It doesn't last, it adds unnecessary weight and it creates a bigger problem for whoever does the next roofing job (probably you). Most importantly, you can't see problems with the sheathing if you don't remove the old roofing.
In my opinion you must tear off the existing roofing. Whether you go back with steel or shingles is your business. If you decide to go with shingles, do yourself a favor and sheath over the existing boards with 7/16" OSB. Not only does it make a better surface for nailing, it also reduces the likelihood you'll fall through the roof. Go over the new sheathing with decent architectural shingles and that roof will be around long after you and I have left this vale of tears.