Normally if you have a single channel IR transmitter from each monitor, you will get interference problems when also using a single channel Ir headphone..
Heres why;
Left monitor 2.3-2-8mhz,
Righ monitor 2.3-2.8mhz,
Headphone is also 2.3-2.8mhz
Left channel is 2.3
Right channel is 2.8
Heres the problem, 2 monitors outputting the same channel at the same frequency, user can be sitting behind left or right monitor or in the centre of both, but the headphones are picking up the same signal from both monitors at the same frequency, these frequencies are not "tuned" 100% and have a torlerance thus it will pick up a clear signal from 1 and some "fuzzy" signal from the other monitor.
To solve this problem, newer installs have monitors with dual frequency ir transmitters and IR headphones with dual channels.
So if you are now sitting behind the left monitor which may be 2.3-2.8, the dual channel headphone can be selected to this frequency, the right monitor is on 3.2-3.8 and sitting behind this the headphone will be set on 3.2-3.8. thus each headphone will only be picking up from a single transmitter, even if the user is sitting between each screen, which ever frequency the headphone is set to, will again only pick up from 1 transmitter, normally built into the monitor itself.
If your system has a frequency selector, make sure both monitors are set differently, providing also you have dual channel IR headphones.
As for the battery life, some IR headphones have battery saving mode, so when no audio signal is present for a set time they will auto power off, these headphones normally have a push on/off type switch opposed to a slide on/off switch.
IR headphones should give you the most battery life in comparison with RF,and bluetooth.