While it might be unclear how or if wearing multiple pieces of Martyr gear would work, it is very clear that at the very least the effect should remain unchanged even if it is not enhanced further.

However, wearing two or more pieces of martyr gear results in no discernible time slow when the message appears saying the effect has been triggered.

I have reproduced this numerous times in the testing cell qasmoke, spawning in enemies while already at low health. When wearing precisely one piece of martyr gear, time slows down as normal, when wearing any other number of gear, time is unchanged.