Two-bath fixing is generally recommended for fiber-base prints. It is not necessary for film and RC papers.
The idea is to extend fixer life by dividing it into two baths. The first bath does most of the fixing and ends up with most of the fixer by-products. The second bath finishes up the fixing and stays relatively pure, thus ensuring adequate fixing without having to change fixer so often.
The technique is: Make two fixing baths. Fix fiber-base prints in the first fix for half the total fixing time, drain and then fix in the second bath for the second half of fixing time. When throughput capacity for the volume of the first fixing bath has been reached (usually 30-40 8x10-inch prints per liter), discard the first bath, move the second bath into first-bath position and mix a new second bath. You can continue this cycle through five changes before needing to mix both baths new.
Hope this helps.
Doremus