Defer to the one who wasn't unsighted. Or have a linesman for the 18 yard box and one for the rest, but on the same side - so only one is active at any given point.
Instead of extra linesmen, you could maybe go for a ref in each half like they have in hockey - less pressure on a single ref and more precise rules needed around currently subjective options e.g. when a card is shown.
The point is that there are non-technical solutions available to the multitude of issues that have arisen whilst trying to implement VAR.
VAR itself was introduced before the technology could cope and we now have the added luxury of incompetent officials running it.
Technology for absolutes - when it works - and then introduce something else if it's actually needed.