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Would this even be possible to write a linker that could link 64bit and 32bit object files?

elf32 + elf64 -> elf32 || elf64

Not sure... but I do link my elf64 object files into an elf32 for my multiboot bootloader. It hasn't given me any trouble.

what linker do you use? ld won't do that...

GNU ld. Works fine with a linker script.

Here's an excerpt from mine:

OUTPUT_FORMAT(elf32-i386)
OUTPUT_ARCH(i386:x86-64)

I can even put 32-bit assembly code in there (and 16-bit with some tweaks).

Awesome, could this be used as a technique to do boot a kernel with grub? I might want to add this to add this to the wiki...

I guess so. The one problem I can see is that everything has to be identity mapped. This isn't a problem for a bootloader, but a long mode kernel would usually be loaded in the higher half, and grub won't be about to do that (unless you have memory up to 0xFFFF800000000000). The only reason I bother with elf64 -> elf32 instead of elf32 -> elf32 in my bootloader is that 64-bit C code needs to be compiled to elf64.

My kernel is not higher half, so it works for me. I'm gonna attempt it right now so let's see what happens.

Oh, this is just converting a 64 bit to 32 bit elf not mixed linking?

I was linking a series of elf64 files into an elf32 using ld. No objcopy or anything like that. You might be able to link elf64 and elf32 files together by doing it in 3 stages: link the elf32s into one relocatable elf32 file, link the elf64s into one relocatable elf32 file, then link the two relocatable elf32s into another elf32.