hi michael

an easy way to determine if your fixer is used to capacity, or nearly to capacity ( i am sure some will tell me i am wrong )
is to do a clip test ...
take your favorite film and open it in daylight in your darkroom so it is totally exposed to light ...
clip off about 1 square inch of film.
put that film in your fixer ( lights on ) and watch the clock. when it clears to base note the time.
double that time for your film fix time .. so if it is 30 seconds ... your fix time for this film is 60 seconds.
split it between 2 baths ... 30 seconds in each ...
paper .. fix it to your manufacturer's recommendations ... split it again between your 2 baths ...

when your fixer takes 2x the original "fresh fix" clip test ( so 1 min in our example ) your fixer is spent and shouldn't be used anymore ...

remove #1 for recovery ( or hauling or whatever your situation is ) make your #2 your new #1 and make a NEW second bath.
do your clip test whenever you process film / print so you know if you are spent &c ..

the thing with 2 baths is the first bath does most of the heavy lifting, and bath 2 does clean-up ...

"the drops" ( like edwal's hypo check ) will give you a false reading, unless you are using edwal's fixer, and often times
people don't really take out 1oz put a the drops in and swirl it to see if it is reabsorbed so they get a false reading that way too ...

the thing about the fix test is to use the same film you always use so the silver content of the film won't be different between the test and your film ...

good luck !
john