Should I maintain portfolio allocation based on cost basis or current value?

I feel the standard advice is to use current valuation while maintaining allocation.

But take an example of 10% allocation in stock A and 10% in stock B. Suppose stock A goes up by 100% and stock B goes down by 50%.

By this rule, I am suppose to sell 50% of stock A and double-down on stock B. I feel it's a terrible idea since I am selling my winners and buying losers.

So I feel cost basis based allocation is a better solution.

Comments?