I use on-board 1G NIC for management and SolarFlare S6102 NIC for data traffic.
No issues so far...
Old implementation: Both ports in LACP connected to same CRS309.
Current operation: Both ports in LACP, but only one is connected to CRS309.
In-progress implementation: Both ports in LACP, each connected to one member of CRS317 MLAG pair.
The only "surprise" was with newest implementation. MLAG was more dead then alive after upgrading one CRS317 to v7.18 while the other was still on v7.17.