Scaffolding would be a much safer option, but wouldn't help with your
knee problem. I think it would be very difficult to get good stability
with a plank and stepladders, and on a plank you have to rely on agility
that your knees may no longer have. I do use a 1x12 on an articulated
ladder to work on my boat, but I am not going very high to do that. The
articulated ladder supports the board about every three feet. For a
longer span, I would want a plank (the planks I have seen flex far less
than dimensional lumber, but I suppose adding a strengthening rail to a
2x12 would give good stability, too. The advantage, to me, of a plank
is that the length can be varied to fit the job, or the storage area.

Renting an elevating platform would be your best solution as you could
ride up and down to just where you want to be. I think some can even be
driven along while you are elevated, but I'd rather ride down and check
the footing when you have to move more than the length of the platform.
A cherry picker would also work, but I think their rental is higher.

al wrote:
Started doing some repairs and painting of fascia and soffit but my
knees can't take going up and down a ladder. Would a 1X12X12 between
two step ladders work alright for scaffolding?