Bandpass filters do an excellent job, for starters. I guess of the commercial
ham filters, I.C.E. and Dunestar have been the main players for some years, and
now the W3NQN filters are available through WX0B and apparently have a superior
reputation for effectiveness. Further knock down the "offending" signal with
stubs in addition to the filters. Filters have been sufficient at WP3R, using
1500 watts, if the two radios are on different bands.
At the 38-transmitter W3AO Field Day (since someone mentioned FD), we have as
many as three transmitters on a band (e.g. CW, SSB and RTTY). Theoretically
you could even have four on one of the bands -- CW, SSB, RTTY and "GOTA," what
used to be called the novice/tech station. We haven't usually had major
problems, but we're only using 150 watts maximum. In fact, the problems are
minor, with quirks some years where some band is having bad problems -- we
suspect those are due to some freak interaction and with a different setup the
next time the problem isn't there.
73 - Rich, KE3Q