I know both Llanfairpwll and Valley stations, and cannot understand on what basis either has had to close. Both platforms are I think pretty straight, so no bends to cause problems, and both should easily be long enough to open at least two carriage doors, and I would have thought more than that. As they are also the most used of the intermediate Anglesey stations, this decision seems to be utterly bizarre.
By two carriage doors, do you mean "all the doors on two carriages" or "both doors on a single carriage". If the latter, then perhaps the problem is the rolling stock - I'm not sure if any of the TfW fleet have Selective Door Openning (
SDO▸ ) systems (except perhaps the
INTERCITY mark 4 carriages which I think I read were having it fitted). If there is no SDO, that means the guard has a choice of unlocking ALL the doors on that side the train (which in most cases will be longer than 1 carraige, although TfW do seem to have a large number of class 153s at the moment) or just one single door, that single door being the door that the guard is standing at and therefore unable to socially distance.
I don't know if TfW's fleet has SDO or not, but if it doesn't and the platforms are only 1 carriage long then TfW would only be able to serve those stations by running single-car class 153s on all services that call.