HIDs really need a stable power supply. its not exactly ideal running them straigh from standard wiring.
you either have a poor / unstable power supply or a faulty ballast.
harnesses are not that hard to knock together, I've made my own several times now. you should be able to make one harness that controls both units so just needed one fuse holder and relay although I've always prefered to keep them seperate.
a couple of automotive relays from maplin should do it, you can get the fuse holders from there also. you should try running them off a harness, its not that much work, could knock one together in probably 15mins if I wanted to however I wouldnt be suprised if on this occassion the ballast is faulty.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso