"My understanding that both of these frequencies should have the same channels, and you should just tune into the one that gives you the best reception..." Each system uses it's own unique frequencies for control channels and voice channels. However, the Talk Groups are the same on both systems.
"I'm pretty sure that the WEST simulcast has more talk groups (Lynnwood PD is all the chatter tonight), but when I'm on the EAST simulcast... I don't hear anything from that talk group (and others)." Correct. Not all Talk Groups are active on both systems. And it changes from time to time. e.g. One day you may hear (insert TG here), and the next day it could be gone. There are some core TG's that seem to always be on both systems. Such as, but not limited to, all the Fire TG's and all the SCSO TG's.
"So, is this normal? Any good way to get them "all" without having my scanner hoping back and forth between EAST and WEST? Should I just tune to west? Are there likely talk groups I'd be missing on the EAST feed then?" Yes it's normal. No, if you want 100% insurance you will hear EVERYTHING, then you need to scan both the East and West system. As mention before, the West system seems to have the largest pool of full time active TG's. Yes, some TG's don't seem to be on the East system all the time. But again, it's random for a number of reasons.
_________________ 
|