No I don't think there is (unless another emu supports it as an extra) however there is a ZX81 emu for the ZX spectrum and also for the AtariST, you could try those for some dual emulation fun.
Hummm I think it's the key of your problem here
let me explain you, as craigix said the ZX81 emu don't exist (yet), but you can surely try a ZX Spectrum emu (in this case the last & really great Speccyal'K emu) to run a ZX81 emu for Spectrum (an emu under an emu in fact :lol: )
as the ZX81 is not too greedy in power I think you can try & maybe success!
but this solution can be also try under CaSTaway
now the pb is to find these ZX81 emu for Atari St or Spectrum ....