In 2005 when AMD came out with an affordable x86_64 processor, I bought one and built my main workstation with it. At the time, there was no x86_64 distros that I knew of, so I installed Debian testing i386 and did a cross compile to add a x86_64 kernel on it. It has worked flawlessly ever since.
I still have that system (I'm typing this on it), even though I've changed everything from the motherboard, RAM, CPU, Hard disks, and power-supply. But I've never reinstalled the system. (If you are wondering about how I did that and changed the HDs, I have a RAID 1 setup, and changing HDs is just a matter of swapping one at a time and doing a resync).
Today, I no longer do a cross compile to build my kernels for it. It handles building a x86_64 kernel just fine, even though my userspace is still i386. And yes, it comes with a x86_64 kernel:
I believe the 'Multi-arch' is to run both x86_64 and x86_32 userspace. But has nothing to do with the kernel (although you need an x86_64 kernel to run x86_64 userspace apps). But if you only have x86_32 userspace, then all you need is that linux-image and you're good to go. Debian has supported this for a while.