AS5400 supports H.323 and SIP dial-peer at the same time without problems.
I've used AS5400 in IP2IP gateway mode to convert SIP in H.323 and vice versa with about 150 concurrent calls.
In lab I also tested SIP, H.323 and MGCP at same time.
In default configuration SIP and H.323 are both active.
AS5400 uses H.323 like default signalling protocol. Is sufficient create a voip dial-peer. To specify SIP you must use the command "session protocol sipv2" under a dial-peer.
To shut down SIP use
voice service voip
sip
call service stop
To shut down H.323 use
no gateway