How much valium are we talking here over these 5 months? There's a big difference between 200mg/day for 5 years and 10mg/day for 6 months. Kicking from the former (or anything around there) is dangerous, around the latter ... not so much. And how long/how much is your current continuous opioid bender?
Obviously the frank and honest discussion w/the doc is a good idea, but personally if it were me I'd shoot for weaning down and kicking the benzo's first, unless your dosage is real low on those and real high on the opioids and/or there's other mitigating factors (like one is 'legal' and one is not).
Have you ever just run out of the valium and gone without for a couple days? If so, how uncomfortable has the experience been?
My guess also is that staying on either one is going to make the detox of the other less unpleasant, particularly having the valium for the opioid withdrawals. However, you miss the opportunity to just 'get it all over with', which there's something to be said for if you can pull it off, and do so safely.
A LOT of what I'd personally suggest is possible/not possible and/or dangerous ... would be dictated by your dosages and how long you've been at it in general, and how long at those dosages.