zed, there's nothing strange about what you want.
The Brits have always favored the large-caliber double rifle for hunting or defending against dangerous animals, especially in Africa. Those who can afford it, still do -- they're generally very, very expensive. They point and acquire a target quickly and reliably, and they're good for shooting at moving targets, just like a SxS shotgun.
What you're really asking for is something that will work like a double rifle, but won't cost $10,000, and will shoot commonly-available slugs instead of expensive and rare Rigby or H&H rounds.
There are a couple of double rifles that aren't too expensive. One is the .45-70 Spartan SPR22, imported by Remington. It's a Russian Baikal, formerly imported by EAA as mentioned by sm (aside to sm: look at Remington's web site for most of the EAA imports). From all reports, Remington's involvement has led to significant improvements in quality, well worth the somewhat higher prices than EAA charged.
http://www.remington.com/products/firearms/centerfire_rifles/SPR_models/SPR22_specs.asp
Their SxS shotguns are remarkably inexpensive. They might work, but their beads will limit you to closer ranges than rifle sights. I think Carlson makes aftermarket chokes for them. The factory chokes may not like slugs, but the barrels ought to be fine. Ask Remington! They have good customer service. Don't blow yourself up based on my advice. Call them.
http://www.remington.com/products/firearms/shotguns/SPR_models/SPR220_side-by-side_specs.asp
Another double rifle, if you can find it, is the Pedersoli. Muzzleloaders are easy to find, but the break-action versio takes some digging.
