Believe it or not, you should be able to split the signal between both items.
If you wire it in and they don't work, then switch the wires going in to one of the 2 (either ECU or speedo) and see if that doesn't fix it. I know polarity shouldn't matter, but I think you'll find that it should work.
Worst case, you may need a module from Dakota Digital or something to run the speedo.