hi -
I do not use igate-group and am able to perform crossband pretty close to what you are wanting to do.
# 144.390 1200 baud
<interface>
serial-device /dev/ttyACM0 57600 8n1 KISS
tx-ok true
callsign N1SU-2
telem-to-is true
</interface>
<interface>
serial-device /dev/ttyAMA0 19200 8n1 KISS
tx-ok true
telem-to-is true
callsign N1SU-4
</interface>
# 144.390 1200 baud
<digipeater>
transmitter N1SU-2
<source>
source N1SU-2
relay-type digipeated
</source>
<source>
source N1SU-4
relay-type digipeated
</source>
<source>
source APRSIS
relay-type third-party
</source>
</digipeater>
<digipeater>
transmitter N1SU-4
<source>
source N1SU-4
relay-type digipeated
</source>
<source>
source N1SU-2
relay-type directonly
</source>
<source>
source APRSIS
relay-type third-party
</source>
</digipeater>
I have some extra parameters in each <digipeater> section (delay, filter, etc.) but you get the idea. What i'm doing may be the incorrect approach but it does exactly what I want and probably will do what you're wanting to do. Note that I use directonly for 2m->440 as personal preference.
hope this helps.
73 Jason N1SU