I agree with Eric Ozrelic's last sentence - if you provide geographic details we may be able to suggest a workable configuration, otherwise we can only provide 'generic' information and guesses.
In your '3 up hill, 3 down hill' scenario with only two channels, without any deeper understanding of the actual situation I would try
A B A
B A B
with sync. (you'll never want two APs to be able to see each other on the same channel withOUT sync, they'll stomp all over each other)
My concern with Eric's suggestion is that if an SM can see two APs on the same channel they will interfere with that SM's downlink, and if an AP can see its own clients PLUS any client of another AP on the same channel then those other-sector SMs would interfere with that AP's uplink.
If the geography is such that the APs could see each other (up and down the mountain) but NOT other-sector SMs, and the geography and/or antenna choice/placement keeps the SMs from seeing multiple APs anywhere near the same signal strength, then his suggestion should work. But an SM that can see a -45dB on AP1 and -51dB on AP2 (on the same freq) likely has an effective SNR of 5-6dB when registering to AP1...
Another benefit with sync would be where two SMs on different APs can see each other. We have multiple instances where two clients roughly across the street or next to each other actually ended up working best aimed at towers behind the other - without sync those two SMs will stomp on each other whenever they utilize uplink. With sync, they will each only listen for downlink data during the same timeslot, and when one is transmitting uplink data the other will either be idle awaiting the next downlink timeslot, OR transmitting itself to it's own AP. End result is they should never actually 'see' each other.
On a tangent, are you planning two channels because of other nearby usage in the band, or because you're going with two 20MHz wide channels? I'm curious because if your clients' bandwidth needs can be met with 10MHz channels you'd have far more flexibility, with 5 available. (4 if you want guard bands, but PMP450 supposedly doesn't require them)
If the only problem with 10MHz would be client capacity rather than individual client max speeds then you could overlay a second AP on a different channel in any sector that starts to overload and split the sector load. (Looking forward, if you're US-FCC more channels should become usable from 3.55GHz to 3.65GHz soon)
j