Update:
Since my last post we have moved to our new location.
There are quite a few things that I have learned in the past weeks concerning multi wan, PPPoE and PPPoA, ML-PPP and DSL in general.
With Podliarius' information/translation I was able to convince my provider that the setup as proposed would work.
Unfortunately, we ran into quite a few snags along the way.
We purchased two TD8816 modems, but these we unable to provide the required bridging.
RFC1483 is not the same as PPPoA -> PPPoE media conversion. PPPoE only works if the ISP actually has the protocol running somewhere.
In the case of our ISP as I understand it, everything is pure ATM until past the DSLAM.
Instead, we needed modems that were able to masquerade as PPPoA client in a transparant fashion, while providing a PPPoE server on the router's side.
Searching this forum, I came across a post by Stephenw10 referring to a specific modem capable of PPPoA -> PPPoE translation.
We purchased two of these modems and pfsense was able to connect to our ISP through them.
However, ML-PPP does not seem to be active. We contacted our ISP and they insist that they have explicitly enabled ML-PPP on their end.
In the PPP log, I see a lot of chatter, but I am unable to determine if pfSense is even attempting to connect with ML-PPP.
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM c29a637b
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #175
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM c29a637b
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #174
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM c29a637b
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #173
ppp: [wan_link1] LCP: state change Starting --> Req-Sent
ppp: [wan_link1] LCP: Up event
ppp: [wan_link1] Link: UP event
ppp: [wan_link1] PPPoE: connection successful
ppp: PPPoE: rec'd ACNAME "Vigor2000 PPPoE"
ppp: [wan_link1] PPPoE: Connecting to ''
ppp: [wan_link1] Link: reconnection attempt 1630
ppp: [wan_link1] Link: reconnection attempt 1630 in 2 seconds
ppp: [wan_link1] LCP: LayerStart
ppp: [wan_link1] LCP: state change Stopped --> Starting
ppp: [wan_link1] LCP: Down event
ppp: [wan_link1] Link: DOWN event
ppp: [wan_link1] PPPoE: connection closed
ppp: [wan_link1] LCP: LayerFinish
ppp: [wan_link1] LCP: state change Req-Sent --> Stopped
ppp: [wan_link1] LCP: parameter negotiation failed
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #172
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #171
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #170
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #169
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #168
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #167
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #166
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #165
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #164
ppp: [wan_link1] ENDPOINTDISC [802.1] 00 e0 2b 89 f2 28
ppp: [wan_link1] MP SHORTSEQ
ppp: [wan_link1] MP MRRU 2048
ppp: [wan_link1] MAGICNUM 3be9b6e4
ppp: [wan_link1] MRU 1492
ppp: [wan_link1] PROTOCOMP
ppp: [wan_link1] LCP: SendConfigReq #163
ppp: [wan_link1] LCP: state change Starting --> Req-Sent
ppp: [wan_link1] LCP: Up event
ppp: [wan_link1] Link: UP event
ppp: [wan_link1] PPPoE: connection successful
ppp: PPPoE: rec'd ACNAME "Vigor2000 PPPoE"
ppp: [wan_link1] PPPoE: Connecting to ''
ppp: [wan_link1] Link: reconnection attempt 1629
ppp: [wan_link1] Link: reconnection attempt 1629 in 4 seconds
ppp: [wan_link1] LCP: LayerStart
ppp: [wan_link1] LCP: state change Stopped --> Starting
Regardless of ML-PPP, both DSL connections work and I can choose which IP I want to use on WAN by using the login data for either DSL connection.
So, we are three quarters of the way there.
Can someone help me figure out if there is something wrong on my end or if there is something my ISP needs to set up?