510-A Safety Code for Model Aircraft Gas Turbines
#5 under Airframe Requirements: Enclosed multiple engines must be segregated in separate pods or partitions where exhaust gases cannot mix causing cross-ignition.
This rule is subject to some interpretation and was discussed at great length in the Kenny Falconer F-18 build thread.
http://www.rcuniverse.com/forum/m_18..._3/key_/tm.htm
If you want an event CD to have a "legal reason" to ban your twin JetJoe powered ARF to fly, then just leave the bypasses out.