Safest way to run 2 6500 radios - each with it's own computer - each connected to an Amp (ALS 600 &

What do you mean "safest"?  I'm not sure what the concern is.

You can tell each computer which radio to use -- read this post

Ahhh OK. Well from a safety standpoint, it's more of a system problem. Ideally you will want to ensure that you have no more than 5mW of aggregate power into any antenna connector when the radio is transmitting. This is peak power of all carriers, not just a single carrier. Assuming that a carrier generated by your local station would be more than 10dB stonger than the peak power from all other stations combined (generally a safe assumption), you could generate a CW carrier with one transmitter and measure the coax going to the receiver with a power meter. You will probably need a low-power meter that reads in the uW to low W range (put your Bird away...). You will also want to test all cross-band/antenna combinations that make sense. If you find a non-compliant set then let's discuss how to solve the problem. For example, if the issue is TX on your tri-blander created a +25dBm signal in your receiver listening to a vertical, more height to your beam. If hit be the answer, Etc...