If its telling you the second lambda isn't ready, I'd tend towards the fact that is the generic message for the data not being available, which it isn't. Which doesn't necessarily cause you problems.
Two things -
If this is in fact a dealer-fit programmed ECU then we can safely assume its been done by someone who knew the correct programming in order to suit the loom and lambda setup. I can see no real case in which someone would take a MUFF car with ECU failure and replace it with something that wasn't also a MUFF ECU. If they've done so, you may assume they knew what they were doing, ie. they're a Ford main dealer. Or they don't know what they're doing so they've sourced the PO51 in the hope that it will work, which I don't buy for two reasons. Firstly MUFF are not rare and secondly PO51 are. If you were looking to replace an ECU its actually more likely you'll find the right one than the wrong one (unless you're looking for later stuff in which case TAPE are not as common on the open market as they sell quicker, but you'd therefore probably wait for another TAPE and not buy a QQP0 which is the later programmable stand-in unit). I'll happily stand corrected but it looks right to me.
Secondly if the ECU wanted a reading and wasn't getting one, you'd probably have a code. I forget the exact P number but I'm quite sure if you got a twin lambda car and unplugged the second one you wouldn't get away with anything not stored.
I'll try it with a twin lambda car if I get a minute to satisfy my curiosity. You may also get a proof of concept by unplugging the first sensor and seeing if you get the same message in the reader about, in conjunction with a fault code. That would tell me the absence of the fault regarding the second one is not a worry.