Develop a SIP&RTP display filter

Hi,

I'm trying to find out the way to make a display filter that includes two different, but related, protocols. I mean, SIP messages contains information about RTP ports. What I am doing now is:

1. Filter a SIP flow
2. Analyse which ports RTP is using
3. Add this information to the display filter
4. Filter again.

I'd like to develop a filter to do it automatically but I've seen nothing similar. Should I develop a chained-dissector, a post-dissector or should I develop inside display filter?

I've taken a look at this: http://wiki.wireshark.org/Lua/Examples#Dump_VoIP_calls_into_separate_files

Thanks