Hello David,
Each interface/sub-interface of an ASA firewall must remain on it's own dedicated Broadcast domain so this is not possible.
If you want to interfaces to sit in the same broadcast domain you could use the Firewall in transparent mode and confiure a BVI.
Looking for some Networking Assistance?
Contact me directly at jcarvaja@laguiadelnetworking.com
I will fix your problem ASAP.
Cheers,
Julio Carvajal Segura
http://laguiadelnetworking.com
Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC