Any operation of MAX north of the river would be a joint effort between TriMet and CTRAN, and CTRAN would be responsible for figuring out what to do with any potential MAX extensions beyond the bridge.
This is news to me. Everything I've seen indicates that a MAX extension was feared by Clark County officials because of how the leaders of MAX might begin to influence planning decisions north of the river. I've never seen anything that said something about joint efforts beyond basic things like getting it built.
It's all in here somewhere
https://www.wsdot.wa.gov/accountability/ssb5806/docs/8_Operations_And_Maintenance/CTRAN_TRIMETIGA_9_26_13.pdf