The way I understand the wiki to EDGE and the attached schematic I would say it is finally still 2G as 3G starts at least with UMTS.
Anyway I believe that the FP2 software won’t allow any other network combination for dual-SIM than 2G + any other for the second card.
You may keep both on 2G, generally that’s possible (there is some switching going on then as the FP2 is designed as DSDS (dual SIM - dual standby, which means only one card/carrier at a time can utilize the discrete modem hardware).
But as there is only one discrete transceiver installed as well, not both cards can be set up for a higher data speed connection, which starts with 3G. As a data link usually is permanent not both cards can occupy the transceiver hardware.
Actually I am not aware if there are phones that can have both cards set to 3G or higher simultaneously.
For phoning via 2G that’s possible, but not very common. These are then DSDA (dual sim - dual active mobiles). Two individual calls at the same time.