A pre-amp with multiple simultaneous outputs.

The manual for my PS Audio PCA-2 explicitly mentions using multiple outputs at once, and it works fine (I'm using one output to main amp, one to subs). I've also used multiple outputs simultaneously on my Bryston BP-25 (two sets unbalanced, one balanced). I've also used high-quality Y-adapters to split a single output, again with no problems. Generally, the only issue is that the amp (or 2nd system) inputs be of reasonably high impedance.

For your application, you may want to find a preamp with a buffered tape output, and use that instead - that way, volume in the 2nd system will be affected only by the local volume control, not by the main preamp volume control. I've actually done this, but not with such a long line.

You may want to use a tape output and a line driver. Van Alstine makes one, and if you go to balanced lines (maybe a good thing with such long runs) there are a number of relatively inexpensive pro-audio options for a line driver.

Finally, I'm now accomplishing something similar to what you want to do by using a Ramsey FM transmitter from the tape out of the upstairs system, and tuning in downstairs. Pretty good sound quality, certainly fine for background listening downstairs.
Rich

I would really suggest a pre-amp with multi-zone capabilities. If theater isn't involved, the adcom is a really inexpensive option. Be careful with home theater receivers with multi-zone outputs, many don't do everything you want. Niles makes a much more expensive version of the same thing.

The other option since you have systems in two other locations: Use the multi-room outputs with a pair of "Y" adaptors. You can run this line level to each pre-amp using CAT-5 wire. You may want to consider hooking up a remote eye at each end also so you can control this main pre-amp with a remote in each room?

Linn has all this figured out to perfection.