Both of your SPA3102 units should work behind the same router, however it may be prudent to move the SIP and RTP ports of one unit to avoid overlap with the other unit.
It's also easy to configure the ATAs so that no port forwarding is required. This is done by setting
NAT Keep Alive Enable to yes, and set
NAT Keep Alive Intvl to 60, and all the
NAT Support Parameters to be as shown here:
<Handle_VIA_received>yes
<Handle_VIA_rport>yes
<Insert_VIA_received>yes
<Insert_VIA_rport>yes
<Substitute_VIA_Addr>yes
<Send_Resp_To_Src_Port>yes
<STUN_Enable>yes
<STUN_Test_Enable>yes
<STUN_Server>stun01.sipphone.com:3478
Depending on how long your router will remember the NAT mapping, you can increase
NAT Keep Alive Intvl, up to 300 for some routers.
The settings that will work behind a NAT router without need for port forwarding are in this post
HERE. Just change the Proxy, User ID, and Password to match your FWD account.
At the above link, you would select one of the Basic Settings Files labeled "For ATAs on a LAN behind a NAT router". If it doesn't load according to the instructions there, you can enter the settings by hand.
You will probably want to improve your dial plan. In order to call PSTN numbers you will need to buy outbound minutes from someplace, except for calling toll free numbers. The dial plan
HERE will get you started, or at least give you some ideas about making your own dial plan.