"The splitters usually do not have any resistors inside.
You just parallel the headphones.
To check just plug in one headphone and while listening plug in the other.
When your amp is low output R the sound may be just slightly softer.
When your amp has a higher output R the headphone will sound noticeably softer.
When you have a multimmeter and a headphone plug you could also measure it.
You will be loading the source with a 16 Ohm load instead of the 32 of 1 headphone,. Most amps won't mind that much, some sources might not play as loud or may show some bass roll-off depending on the design.
Audio memory lasts about 1 second. After a second of 'pause' the brain already recalibrates.
Having a reference is always better than having none so think it is a way better way to listen to mods than other methods.
I often use a HD650 on Kameleon to 'reset' my head when comparing headphones."