T. Calderone has it right.. Valium, is much better to use for tapering than using opiates, like H, which will just cause either another addiction to start up, when unnecessary, while you could just use a much longer acting, less potent benzo to help with the Xanax WD or cause like you said, to go into both drugs' withdrawls, which you don't want to do.. See, valium has a much longer half-life and duration of action than Xanax does, and heroin is not really meant for benzo WD, although it does help, just wont help for long once you fully try to get off Xanax and stop taking them completely, not just tapering.. I see where your coming from using it to ease the anxiety/physical discomfort, but in the long run, unless you plan on getting another 1,000 pound gorilla on your back with Heroin addiction, it wont help benzo WD.. only benzos(longer acting ones, valium is the best though) can help it, and many other prescribed/OTC non-narcotic meds... It still sucks fucking asshole with them! Trust me, been there done that...
I have gone through relatively high dose, of daily klonopin and subutex WD at the same time more than once.. as I am prescribed both of those and have run out of them early, lost them and been unable to fill my script/get a refill to soon more than a few times and it was fucking hell, even with "comfort" meds, which really did next to nothing going through those 2 drugs WD together.. since the last time I did that, I have made it a personal goal to never let myself get into that situation again, until the dreadful day comes where I have to get off them, if it ever does... right now, I am absolutely terrified at the thought of ever having to kick klonopin, and plan on being on it forever! I need it anyways as I have severe anxiety both general/social, so that doesn't bother me! Don't know when I will ever get off bupe either, don't even wanna think about it tbh!
Please, be careful! Good luck man. Hope this WD/taper goes well for you!